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class="sidebar-link">ArrayBuffer</a></li><li><a href="/blog/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/blog/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/blog/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/blog/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/blog/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/blog/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06225672><div class="articleInfo" data-v-06225672><ul class="breadcrumbs" data-v-06225672><li data-v-06225672><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06225672></a></li> <li data-v-06225672><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06225672>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06225672><div title="作者" class="author iconfont icon-touxiang" data-v-06225672><a href="javascript:;" data-v-06225672>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06225672><a href="javascript:;" data-v-06225672>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,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">数组的扩展<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="数组的扩展"><a href="#数组的扩展" class="header-anchor">#</a> 数组的扩展</h1> <h2 id="扩展运算符"><a href="#扩展运算符" class="header-anchor">#</a> 扩展运算符</h2> <h3 id="含义"><a href="#含义" class="header-anchor">#</a> 含义</h3> <p>扩展运算符（spread）是三个点（<code>...</code>）。它好比 rest 参数的逆运算，<strong>将一个数组转为用逗号分隔的参数序列</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token comment">// 1 2 3</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">)</span>
<span class="token comment">// 1 2 3 4 5</span>

<span class="token punctuation">[</span><span class="token operator">...</span>document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'div'</span><span class="token punctuation">)</span><span class="token punctuation">]</span>
<span class="token comment">// [&lt;div&gt;, &lt;div&gt;, &lt;div&gt;]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div> <p><strong>该运算符主要用于函数调用</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">push</span><span class="token punctuation">(</span><span class="token parameter">array<span class="token punctuation">,</span> <span class="token operator">...</span>items</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  array<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token operator">...</span>items<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> numbers <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">38</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token function">add</span><span class="token punctuation">(</span><span class="token operator">...</span>numbers<span class="token punctuation">)</span> <span class="token comment">// 42</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>array.push(...items)</code>和<code>add(...numbers)</code>这两行，都是函数的调用，它们都使用了扩展运算符。该运算符将一个数组，变为参数序列。</p> <p>扩展运算符与正常的函数参数可以结合使用，非常灵活。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter">v<span class="token punctuation">,</span> w<span class="token punctuation">,</span> x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> z</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span>
<span class="token keyword">const</span> args <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token operator">...</span>args<span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>扩展运算符后面还可以放置表达式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span>
  <span class="token operator">...</span><span class="token punctuation">(</span>x <span class="token operator">&gt;</span> <span class="token number">0</span> <span class="token operator">?</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">]</span> <span class="token operator">:</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
  <span class="token string">'b'</span><span class="token punctuation">,</span>
<span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>如果扩展运算符后面是一个空数组，则不产生任何效果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">]</span>
<span class="token comment">// [1]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>注意，只有函数调用时，扩展运算符才可以放在圆括号中，否则会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token comment">// Uncaught SyntaxError: Unexpected number</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token comment">// Uncaught SyntaxError: Unexpected number</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token comment">// 1 2</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面三种情况，扩展运算符都放在圆括号里面，但是前两种情况会报错，因为扩展运算符所在的括号不是函数调用。</p> <h3 id="替代函数的-apply-方法"><a href="#替代函数的-apply-方法" class="header-anchor">#</a> 替代函数的 apply 方法</h3> <p>由于扩展运算符可以展开数组，所以不再需要<code>apply</code>方法，将数组转为函数的参数了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES5 的写法</span>
<span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> z</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>
<span class="token keyword">var</span> args <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token function">f</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> args<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// ES6的写法</span>
<span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> z</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>
<span class="token keyword">let</span> args <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token function">f</span><span class="token punctuation">(</span><span class="token operator">...</span>args<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>下面是扩展运算符取代<code>apply</code>方法的一个实际的例子，应用<code>Math.max</code>方法，简化求出一个数组最大元素的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES5 的写法</span>
Math<span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">14</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">77</span><span class="token punctuation">]</span><span class="token punctuation">)</span>

<span class="token comment">// ES6 的写法</span>
Math<span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token number">14</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">77</span><span class="token punctuation">]</span><span class="token punctuation">)</span>

<span class="token comment">// 等同于</span>
Math<span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span><span class="token number">14</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">77</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，由于 JavaScript 不提供求数组最大元素的函数，所以只能套用<code>Math.max</code>函数，将数组转为一个参数序列，然后求最大值。有了扩展运算符以后，就可以直接用<code>Math.max</code>了。</p> <p>另一个例子是通过<code>push</code>函数，将一个数组添加到另一个数组的尾部。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES5的 写法</span>
<span class="token keyword">var</span> arr1 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> arr2 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>arr1<span class="token punctuation">,</span> arr2<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// ES6 的写法</span>
<span class="token keyword">let</span> arr1 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> arr2 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
arr1<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token operator">...</span>arr2<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码的 ES5 写法中，<code>push</code>方法的参数不能是数组，所以只好通过<code>apply</code>方法变通使用<code>push</code>方法。有了扩展运算符，就可以直接将数组传入<code>push</code>方法。</p> <p>下面是另外一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES5</span>
<span class="token keyword">new</span> <span class="token punctuation">(</span>Date<span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>Date<span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token number">2015</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token comment">// ES6</span>
<span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token number">2015</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h3 id="扩展运算符的应用"><a href="#扩展运算符的应用" class="header-anchor">#</a> 扩展运算符的应用</h3> <p><strong>（1）复制数组</strong></p> <p>数组是复合的数据类型，直接复制的话，只是复制了指向底层数据结构的指针，而不是克隆一个全新的数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> a1 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> a2 <span class="token operator">=</span> a1<span class="token punctuation">;</span>

a2<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
a1 <span class="token comment">// [2, 2]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>a2</code>并不是<code>a1</code>的克隆，而是指向同一份数据的另一个指针。修改<code>a2</code>，会直接导致<code>a1</code>的变化。</p> <p>ES5 只能用变通方法来复制数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> a1 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> a2 <span class="token operator">=</span> a1<span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

a2<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
a1 <span class="token comment">// [1, 2]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>a1</code>会返回原数组的克隆，再修改<code>a2</code>就不会对<code>a1</code>产生影响。</p> <p>扩展运算符提供了复制数组的简便写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> a1 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token comment">// 写法一</span>
<span class="token keyword">const</span> a2 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>a1<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token comment">// 写法二</span>
<span class="token keyword">const</span> <span class="token punctuation">[</span><span class="token operator">...</span>a2<span class="token punctuation">]</span> <span class="token operator">=</span> a1<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面的两种写法，<code>a2</code>都是<code>a1</code>的克隆。</p> <p><strong>（2）合并数组</strong></p> <p>扩展运算符提供了数组合并的新写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> arr1 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> arr2 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> arr3 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'d'</span><span class="token punctuation">,</span> <span class="token string">'e'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token comment">// ES5 的合并数组</span>
arr1<span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span>arr2<span class="token punctuation">,</span> arr3<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// [ 'a', 'b', 'c', 'd', 'e' ]</span>

<span class="token comment">// ES6 的合并数组</span>
<span class="token punctuation">[</span><span class="token operator">...</span>arr1<span class="token punctuation">,</span> <span class="token operator">...</span>arr2<span class="token punctuation">,</span> <span class="token operator">...</span>arr3<span class="token punctuation">]</span>
<span class="token comment">// [ 'a', 'b', 'c', 'd', 'e' ]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>不过，这两种方法都是浅拷贝，使用的时候需要注意。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> a1 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token number">1</span> <span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> a2 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">{</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> a3 <span class="token operator">=</span> a1<span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span>a2<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> a4 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>a1<span class="token punctuation">,</span> <span class="token operator">...</span>a2<span class="token punctuation">]</span><span class="token punctuation">;</span>

a3<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">===</span> a1<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token comment">// true</span>
a4<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">===</span> a1<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>a3</code>和<code>a4</code>是用两种不同方法合并而成的新数组，但是它们的成员都是对原数组成员的引用，这就是浅拷贝。如果修改了引用指向的值，会同步反映到新数组。</p> <p><strong>（3）与解构赋值结合</strong></p> <p>扩展运算符可以与解构赋值结合起来，用于生成数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES5</span>
a <span class="token operator">=</span> list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">,</span> rest <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>
<span class="token comment">// ES6</span>
<span class="token punctuation">[</span>a<span class="token punctuation">,</span> <span class="token operator">...</span>rest<span class="token punctuation">]</span> <span class="token operator">=</span> list
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>下面是另外一些例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token punctuation">[</span>first<span class="token punctuation">,</span> <span class="token operator">...</span>rest<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
first <span class="token comment">// 1</span>
rest  <span class="token comment">// [2, 3, 4, 5]</span>

<span class="token keyword">const</span> <span class="token punctuation">[</span>first<span class="token punctuation">,</span> <span class="token operator">...</span>rest<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
first <span class="token comment">// undefined</span>
rest  <span class="token comment">// []</span>

<span class="token keyword">const</span> <span class="token punctuation">[</span>first<span class="token punctuation">,</span> <span class="token operator">...</span>rest<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">&quot;foo&quot;</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
first  <span class="token comment">// &quot;foo&quot;</span>
rest   <span class="token comment">// []</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>如果将扩展运算符用于数组赋值，只能放在参数的最后一位，否则会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token punctuation">[</span><span class="token operator">...</span>butLast<span class="token punctuation">,</span> last<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token comment">// 报错</span>

<span class="token keyword">const</span> <span class="token punctuation">[</span>first<span class="token punctuation">,</span> <span class="token operator">...</span>middle<span class="token punctuation">,</span> last<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token comment">// 报错</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p><strong>（4）字符串</strong></p> <p>扩展运算符还可以将字符串转为真正的数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token operator">...</span><span class="token string">'hello'</span><span class="token punctuation">]</span>
<span class="token comment">// [ &quot;h&quot;, &quot;e&quot;, &quot;l&quot;, &quot;l&quot;, &quot;o&quot; ]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面的写法，有一个重要的好处，那就是能够正确识别四个字节的 Unicode 字符。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'x\uD83D\uDE80y'</span><span class="token punctuation">.</span>length <span class="token comment">// 4</span>
<span class="token punctuation">[</span><span class="token operator">...</span><span class="token string">'x\uD83D\uDE80y'</span><span class="token punctuation">]</span><span class="token punctuation">.</span>length <span class="token comment">// 3</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码的第一种写法，JavaScript 会将四个字节的 Unicode 字符，识别为 2 个字符，采用扩展运算符就没有这个问题。因此，正确返回字符串长度的函数，可以像下面这样写。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">length</span><span class="token punctuation">(</span><span class="token parameter">str</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token operator">...</span>str<span class="token punctuation">]</span><span class="token punctuation">.</span>length<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">length</span><span class="token punctuation">(</span><span class="token string">'x\uD83D\uDE80y'</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>凡是涉及到操作四个字节的 Unicode 字符的函数，都有这个问题。因此，最好都用扩展运算符改写。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> str <span class="token operator">=</span> <span class="token string">'x\uD83D\uDE80y'</span><span class="token punctuation">;</span>

str<span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reverse</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span>
<span class="token comment">// 'y\uDE80\uD83Dx'</span>

<span class="token punctuation">[</span><span class="token operator">...</span>str<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">reverse</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span>
<span class="token comment">// 'y\uD83D\uDE80x'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，如果不用扩展运算符，字符串的<code>reverse</code>操作就不正确。</p> <p><strong>（5）实现了 Iterator 接口的对象</strong></p> <p>任何定义了遍历器（Iterator）接口的对象（参阅 Iterator 一章），都可以用扩展运算符转为真正的数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> nodeList <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'div'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> array <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>nodeList<span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>querySelectorAll</code>方法返回的是一个<code>NodeList</code>对象。它不是数组，而是一个类似数组的对象。这时，扩展运算符可以将其转为真正的数组，原因就在于<code>NodeList</code>对象实现了 Iterator 。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token class-name">Number</span><span class="token punctuation">.</span>prototype<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
  <span class="token keyword">let</span> num <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">while</span> <span class="token punctuation">(</span>i <span class="token operator">&lt;</span> num<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> i<span class="token operator">++</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token number">.5</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// [0, 1, 2, 3, 4]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，先定义了<code>Number</code>对象的遍历器接口，扩展运算符将<code>5</code>自动转成<code>Number</code>实例以后，就会调用这个接口，就会返回自定义的结果。</p> <p>对于那些没有部署 Iterator 接口的类似数组的对象，扩展运算符就无法将其转为真正的数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arrayLike <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token string-property property">'0'</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span>
  <span class="token string-property property">'1'</span><span class="token operator">:</span> <span class="token string">'b'</span><span class="token punctuation">,</span>
  <span class="token string-property property">'2'</span><span class="token operator">:</span> <span class="token string">'c'</span><span class="token punctuation">,</span>
  <span class="token literal-property property">length</span><span class="token operator">:</span> <span class="token number">3</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// TypeError: Cannot spread non-iterable object.</span>
<span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>arrayLike<span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>arrayLike</code>是一个类似数组的对象，但是没有部署 Iterator 接口，扩展运算符就会报错。这时，可以改为使用<code>Array.from</code>方法将<code>arrayLike</code>转为真正的数组。</p> <p><strong>（6）Map 和 Set 结构，Generator 函数</strong></p> <p>扩展运算符内部调用的是数据结构的 Iterator 接口，因此只要具有 Iterator 接口的对象，都可以使用扩展运算符，比如 Map 结构。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Map</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
  <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token string">'one'</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
  <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token string">'two'</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
  <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token string">'three'</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>map<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// [1, 2, 3]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>Generator 函数运行后，返回一个遍历器对象，因此也可以使用扩展运算符。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token function-variable function">go</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token operator">*</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
  <span class="token keyword">yield</span> <span class="token number">1</span><span class="token punctuation">;</span>
  <span class="token keyword">yield</span> <span class="token number">2</span><span class="token punctuation">;</span>
  <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token punctuation">[</span><span class="token operator">...</span><span class="token function">go</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token comment">// [1, 2, 3]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，变量<code>go</code>是一个 Generator 函数，执行后返回的是一个遍历器对象，对这个遍历器对象执行扩展运算符，就会将内部遍历得到的值，转为一个数组。</p> <p>如果对没有 Iterator 接口的对象，使用扩展运算符，将会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>obj<span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// TypeError: Cannot spread non-iterable object</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="array-from"><a href="#array-from" class="header-anchor">#</a> Array.from()</h2> <p><code>Array.from</code>方法<strong>用于将两类对象转为真正的数组</strong>：<strong>类似数组的对象</strong>（array-like object）和<strong>可遍历（iterable）的对象</strong>（包括 ES6 新增的数据结构 Set 和 Map）。</p> <p>下面是一个类似数组的对象，<code>Array.from</code>将它转为真正的数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arrayLike <span class="token operator">=</span> <span class="token punctuation">{</span>
    <span class="token string-property property">'0'</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span>
    <span class="token string-property property">'1'</span><span class="token operator">:</span> <span class="token string">'b'</span><span class="token punctuation">,</span>
    <span class="token string-property property">'2'</span><span class="token operator">:</span> <span class="token string">'c'</span><span class="token punctuation">,</span>
    <span class="token literal-property property">length</span><span class="token operator">:</span> <span class="token number">3</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// ES5的写法</span>
<span class="token keyword">var</span> arr1 <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>arrayLike<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ['a', 'b', 'c']</span>

<span class="token comment">// ES6的写法</span>
<span class="token keyword">let</span> arr2 <span class="token operator">=</span> Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>arrayLike<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ['a', 'b', 'c']</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>实际应用中，常见的类似数组的对象是 DOM 操作返回的 NodeList 集合，以及函数内部的<code>arguments</code>对象。<code>Array.from</code>都可以将它们转为真正的数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// NodeList对象</span>
<span class="token keyword">let</span> ps <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'p'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>ps<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token parameter">p</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> p<span class="token punctuation">.</span>textContent<span class="token punctuation">.</span>length <span class="token operator">&gt;</span> <span class="token number">100</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// arguments对象</span>
<span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">var</span> args <span class="token operator">=</span> Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>arguments<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>querySelectorAll</code>方法返回的是一个类似数组的对象，可以将这个对象转为真正的数组，再使用<code>filter</code>方法。</p> <p>只要是部署了 Iterator 接口的数据结构，<code>Array.from</code>都能将其转为数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span>
<span class="token comment">// ['h', 'e', 'l', 'l', 'o']</span>

<span class="token keyword">let</span> namesSet <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Set</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>namesSet<span class="token punctuation">)</span> <span class="token comment">// ['a', 'b']</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，字符串和 Set 结构都具有 Iterator 接口，因此可以被<code>Array.from</code>转为真正的数组。</p> <p>如果参数是一个真正的数组，<code>Array.from</code>会返回一个一模一样的新数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token comment">// [1, 2, 3]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>值得提醒的是，<strong>扩展运算符</strong>（<code>...</code>）<strong>也可以将某些数据结构转为数组</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// arguments对象</span>
<span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> args <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>arguments<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// NodeList对象</span>
<span class="token punctuation">[</span><span class="token operator">...</span>document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'div'</span><span class="token punctuation">)</span><span class="token punctuation">]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>扩展运算符背后调用的是遍历器接口（<code>Symbol.iterator</code>），如果一个对象没有部署这个接口，就无法转换。<code>Array.from</code>方法还支持类似数组的对象。所谓类似数组的对象，本质特征只有一点，即必须有<code>length</code>属性。因此，<strong>任何有<code>length</code>属性的对象，都可以通过<code>Array.from</code>方法转为数组</strong>，而此时扩展运算符就无法转换。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">length</span><span class="token operator">:</span> <span class="token number">3</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// [ undefined, undefined, undefined ]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>Array.from</code>返回了一个具有三个成员的数组，每个位置的值都是<code>undefined</code>。扩展运算符转换不了这个对象。</p> <p>对于还没有部署该方法的浏览器，可以用<code>Array.prototype.slice</code>方法替代。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> toArray <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span>
  Array<span class="token punctuation">.</span>from <span class="token operator">?</span> Array<span class="token punctuation">.</span><span class="token function-variable function">from</span> <span class="token operator">:</span> <span class="token parameter">obj</span> <span class="token operator">=&gt;</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>Array.from</code>还可以<strong>接受第二个参数，作用类似于数组的<code>map</code>方法</strong>，用来对每个元素进行处理，将处理后的值放入返回的数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>arrayLike<span class="token punctuation">,</span> <span class="token parameter">x</span> <span class="token operator">=&gt;</span> x <span class="token operator">*</span> x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 等同于</span>
Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>arrayLike<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> x <span class="token operator">*</span> x<span class="token punctuation">)</span><span class="token punctuation">;</span>

Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> x <span class="token operator">*</span> x<span class="token punctuation">)</span>
<span class="token comment">// [1, 4, 9]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>下面的例子是取出一组 DOM 节点的文本内容。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> spans <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'span.name'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// map()</span>
<span class="token keyword">let</span> names1 <span class="token operator">=</span> <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>spans<span class="token punctuation">,</span> <span class="token parameter">s</span> <span class="token operator">=&gt;</span> s<span class="token punctuation">.</span>textContent<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// Array.from()</span>
<span class="token keyword">let</span> names2 <span class="token operator">=</span> Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>spans<span class="token punctuation">,</span> <span class="token parameter">s</span> <span class="token operator">=&gt;</span> s<span class="token punctuation">.</span>textContent<span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>下面的例子将数组中布尔值为<code>false</code>的成员转为<code>0</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> n <span class="token operator">||</span> <span class="token number">0</span><span class="token punctuation">)</span>
<span class="token comment">// [1, 0, 2, 0, 3]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>另一个例子是返回各种数据的类型。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">typesOf</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>arguments<span class="token punctuation">,</span> <span class="token parameter">value</span> <span class="token operator">=&gt;</span> <span class="token keyword">typeof</span> value<span class="token punctuation">)</span>
<span class="token punctuation">}</span>
<span class="token function">typesOf</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token number">NaN</span><span class="token punctuation">)</span>
<span class="token comment">// ['object', 'object', 'number']</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>如果<code>map</code>函数里面用到了<code>this</code>关键字，还可以传入<code>Array.from</code>的<strong>第三个参数，用来绑定<code>this</code>。</strong></p> <p><strong><code>Array.from()</code>可以将各种值转为真正的数组，并且还提供<code>map</code>功能。这实际上意味着，只要有一个原始的数据结构，你就可以先对它的值进行处理，然后转成规范的数组结构，进而就可以使用数量众多的数组方法。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">length</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token string">'jack'</span><span class="token punctuation">)</span>
<span class="token comment">// ['jack', 'jack']</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>Array.from</code>的第一个参数指定了第二个参数运行的次数。这种特性可以让该方法的用法变得非常灵活。</p> <p><code>Array.from()</code>的另一个应用是，将字符串转为数组，然后返回字符串的长度。因为它能正确处理各种 Unicode 字符，可以避免 JavaScript 将大于<code>\uFFFF</code>的 Unicode 字符，算作两个字符的 bug。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">countSymbols</span><span class="token punctuation">(</span><span class="token parameter">string</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>string<span class="token punctuation">)</span><span class="token punctuation">.</span>length<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h2 id="array-of-总是返回参数值组成的数组"><a href="#array-of-总是返回参数值组成的数组" class="header-anchor">#</a> Array.of() 总是返回参数值组成的数组</h2> <p><code>Array.of</code>方法<strong>用于将一组值，转换为数组</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Array<span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">11</span><span class="token punctuation">,</span> <span class="token number">8</span><span class="token punctuation">)</span> <span class="token comment">// [3,11,8]</span>
Array<span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// [3]</span>
Array<span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">.</span>length <span class="token comment">// 1</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>这个方法的主要目的，是弥补数组构造函数<code>Array()</code>的不足。因为参数个数的不同，会导致<code>Array()</code>的行为有差异。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">Array</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// []</span>
<span class="token function">Array</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// [, , ,]</span>
<span class="token function">Array</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">11</span><span class="token punctuation">,</span> <span class="token number">8</span><span class="token punctuation">)</span> <span class="token comment">// [3, 11, 8]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>Array</code>方法没有参数、一个参数、三个参数时，返回结果都不一样。只有当参数个数不少于 2 个时，<code>Array()</code>才会返回由参数组成的新数组。参数个数只有一个时，实际上是指定数组的长度。</p> <p><code>Array.of</code>基本上可以用来替代<code>Array()</code>或<code>new Array()</code>，并且不存在由于参数不同而导致的重载。<strong>它的行为非常统一</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Array<span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// []</span>
Array<span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token comment">// [undefined]</span>
Array<span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// [1]</span>
Array<span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// [1, 2]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><code>Array.of</code><strong>总是返回参数值组成的数组</strong>。如果没有参数，就返回一个空数组。</p> <p><code>Array.of</code>方法可以用下面的代码模拟实现。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">ArrayOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>arguments<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h2 id="数组实例的-copywithin"><a href="#数组实例的-copywithin" class="header-anchor">#</a> 数组实例的 copyWithin()</h2> <p>数组实例的<code>copyWithin()</code>方法，在当前数组内部，将指定位置的成员复制到其他位置（会覆盖原有成员），然后返回当前数组。也就是说，使用这个方法，<strong>会修改当前数组</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">copyWithin</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> start <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">,</span> end <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>length<span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>它接受三个参数。</p> <ul><li>target（必需）：从该位置开始替换数据。如果为负值，表示倒数。</li> <li>start（可选）：从该位置开始读取数据，默认为 0。如果为负值，表示从末尾开始计算。</li> <li>end（可选）：到该位置前停止读取数据，默认等于数组长度。如果为负值，表示从末尾开始计算。</li></ul> <p>这三个参数都应该是数值，如果不是，会自动转为数值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">copyWithin</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span>
<span class="token comment">// [4, 5, 3, 4, 5]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码表示将从 3 号位直到数组结束的成员（4 和 5），复制到从 0 号位开始的位置，结果覆盖了原来的 1 和 2。</p> <p>下面是更多例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 将3号位复制到0号位</span>
<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">copyWithin</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span>
<span class="token comment">// [4, 2, 3, 4, 5]</span>

<span class="token comment">// -2相当于3号位，-1相当于4号位</span>
<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">copyWithin</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span>
<span class="token comment">// [4, 2, 3, 4, 5]</span>

<span class="token comment">// 将3号位复制到0号位</span>
<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">copyWithin</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">length</span><span class="token operator">:</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span>
<span class="token comment">// {0: 1, 3: 1, length: 5}</span>

<span class="token comment">// 将2号位到数组结束，复制到0号位</span>
<span class="token keyword">let</span> i32a <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Int32Array</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
i32a<span class="token punctuation">.</span><span class="token function">copyWithin</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Int32Array [3, 4, 5, 4, 5]</span>

<span class="token comment">// 对于没有部署 TypedArray 的 copyWithin 方法的平台</span>
<span class="token comment">// 需要采用下面的写法</span>
<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">copyWithin</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Int32Array</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Int32Array [4, 2, 3, 4, 5]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><h2 id="数组实例的-find-和-findindex"><a href="#数组实例的-find-和-findindex" class="header-anchor">#</a> 数组实例的 find() 和 findIndex()</h2> <p>数组实例的<code>find</code>方法，用于<strong>找出第一个符合条件的数组成员</strong>。它的参数是一个回调函数，所有数组成员依次执行该回调函数，直到找出第一个返回值为<code>true</code>的成员，然后返回该成员。如果没有符合条件的成员，则返回<code>undefined</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">find</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> n <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span>
<span class="token comment">// -5</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码找出数组中第一个小于 0 的成员。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">,</span> <span class="token number">15</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">find</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">value<span class="token punctuation">,</span> index<span class="token punctuation">,</span> arr</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> value <span class="token operator">&gt;</span> <span class="token number">9</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// 10</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>find</code>方法的回调函数可以接受三个参数，依次为当前的值、当前的位置和原数组。</p> <p>数组实例的<code>findIndex</code>方法的用法与<code>find</code>方法非常类似，<strong>返回第一个符合条件的数组成员的位置</strong>，如果所有成员都不符合条件，则返回<code>-1</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">,</span> <span class="token number">15</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">findIndex</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">value<span class="token punctuation">,</span> index<span class="token punctuation">,</span> arr</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> value <span class="token operator">&gt;</span> <span class="token number">9</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>这两个方法都可以<strong>接受第二个参数</strong>，用来<strong>绑定回调函数的<code>this</code>对象</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter">v</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
  <span class="token keyword">return</span> v <span class="token operator">&gt;</span> <span class="token keyword">this</span><span class="token punctuation">.</span>age<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">let</span> person <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'John'</span><span class="token punctuation">,</span> <span class="token literal-property property">age</span><span class="token operator">:</span> <span class="token number">20</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">[</span><span class="token number">10</span><span class="token punctuation">,</span> <span class="token number">12</span><span class="token punctuation">,</span> <span class="token number">26</span><span class="token punctuation">,</span> <span class="token number">15</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">find</span><span class="token punctuation">(</span>f<span class="token punctuation">,</span> person<span class="token punctuation">)</span><span class="token punctuation">;</span>    <span class="token comment">// 26</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面的代码中，<code>find</code>函数接收了第二个参数<code>person</code>对象，回调函数中的<code>this</code>对象指向<code>person</code>对象。</p> <p>另外，这两个方法都可以发现<code>NaN</code>，弥补了数组的<code>indexOf</code>方法的不足。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">NaN</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span>
<span class="token comment">// -1</span>

<span class="token punctuation">[</span><span class="token number">NaN</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">findIndex</span><span class="token punctuation">(</span><span class="token parameter">y</span> <span class="token operator">=&gt;</span> Object<span class="token punctuation">.</span><span class="token function">is</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">,</span> y<span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token comment">// 0</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>indexOf</code>方法无法识别数组的<code>NaN</code>成员，但是<code>findIndex</code>方法可以借助<code>Object.is</code>方法做到。</p> <h2 id="数组实例的-fill"><a href="#数组实例的-fill" class="header-anchor">#</a> 数组实例的 fill()</h2> <p><code>fill</code>方法使用给定值，填充一个数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">fill</span><span class="token punctuation">(</span><span class="token number">7</span><span class="token punctuation">)</span>
<span class="token comment">// [7, 7, 7]</span>

<span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">fill</span><span class="token punctuation">(</span><span class="token number">7</span><span class="token punctuation">)</span>
<span class="token comment">// [7, 7, 7]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码表明，<code>fill</code>方法用于空数组的初始化非常方便。数组中已有的元素，会被全部抹去。</p> <p><code>fill</code>方法还可以<strong>接受第二个</strong>和<strong>第三个参数</strong>，用于指定填充的<strong>起始位置</strong>和<strong>结束位置</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">fill</span><span class="token punctuation">(</span><span class="token number">7</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span>
<span class="token comment">// ['a', 7, 'c']</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码表示，<code>fill</code>方法从 1 号位开始，向原数组填充 7，到 2 号位之前结束。</p> <p>注意，如果填充的类型为对象，那么被赋值的是同一个内存地址的对象，而不是深拷贝对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">fill</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">&quot;Mike&quot;</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
arr<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token string">&quot;Ben&quot;</span><span class="token punctuation">;</span>
arr
<span class="token comment">// [{name: &quot;Ben&quot;}, {name: &quot;Ben&quot;}, {name: &quot;Ben&quot;}]</span>

<span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">fill</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
arr<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
arr
<span class="token comment">// [[5], [5], [5]]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><h2 id="数组实例的-entries-keys-和-values"><a href="#数组实例的-entries-keys-和-values" class="header-anchor">#</a> 数组实例的 entries()，keys() 和 values()</h2> <p>ES6 提供三个新的方法——<code>entries()</code>，<code>keys()</code>和<code>values()</code>——用于遍历数组。它们都<strong>返回一个遍历器对象</strong>（详见《Iterator》一章），可以用<code>for...of</code>循环进行遍历，唯一的区别是<code>keys()</code>是对<strong>键名</strong>的遍历、<code>values()</code>是对<strong>键值</strong>的遍历，<code>entries()</code>是对<strong>键值对</strong>的遍历。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> index <span class="token keyword">of</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 0</span>
<span class="token comment">// 1</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> elem <span class="token keyword">of</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>elem<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 'a'</span>
<span class="token comment">// 'b'</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span>index<span class="token punctuation">,</span> elem<span class="token punctuation">]</span> <span class="token keyword">of</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">entries</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>index<span class="token punctuation">,</span> elem<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 0 &quot;a&quot;</span>
<span class="token comment">// 1 &quot;b&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>如果不使用<code>for...of</code>循环，可以手动调用遍历器对象的<code>next</code>方法，进行遍历。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> letter <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> entries <span class="token operator">=</span> letter<span class="token punctuation">.</span><span class="token function">entries</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>entries<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [0, 'a']</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>entries<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [1, 'b']</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>entries<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [2, 'c']</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h2 id="数组实例的-includes"><a href="#数组实例的-includes" class="header-anchor">#</a> 数组实例的 includes()</h2> <p><code>Array.prototype.includes</code>方法返回一个布尔值，表示某个数组<strong>是否包含给定的值</strong>，与字符串的<code>includes</code>方法类似。ES2016 引入了该方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">includes</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>     <span class="token comment">// true</span>
<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">includes</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span>     <span class="token comment">// false</span>
<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">NaN</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">includes</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>该方法的第二个参数表示搜索的起始位置，默认为<code>0</code>。如果第二个参数为负数，则表示倒数的位置，如果这时它大于数组长度（比如第二个参数为<code>-4</code>，但数组长度为<code>3</code>），则会重置为从<code>0</code>开始。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">includes</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// false</span>
<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">includes</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>没有该方法之前，我们通常使用数组的<code>indexOf</code>方法，检查是否包含某个值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">if</span> <span class="token punctuation">(</span>arr<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span>el<span class="token punctuation">)</span> <span class="token operator">!==</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>indexOf</code>方法有两个缺点，一是不够语义化，它的含义是找到参数值的第一个出现位置，所以要去比较是否不等于<code>-1</code>，表达起来不够直观。二是，它内部使用严格相等运算符（<code>===</code>）进行判断，这会导致对<code>NaN</code>的误判。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">NaN</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span>
<span class="token comment">// -1</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>includes</code>使用的是不一样的判断算法，就没有这个问题。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">NaN</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">includes</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span>
<span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>下面代码用来检查当前环境是否支持该方法，如果不支持，部署一个简易的替代版本。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> contains <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span>
  <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>includes
    <span class="token operator">?</span> <span class="token punctuation">(</span><span class="token parameter">arr<span class="token punctuation">,</span> value</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> arr<span class="token punctuation">.</span><span class="token function">includes</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
    <span class="token operator">:</span> <span class="token punctuation">(</span><span class="token parameter">arr<span class="token punctuation">,</span> value</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> arr<span class="token punctuation">.</span><span class="token function">some</span><span class="token punctuation">(</span><span class="token parameter">el</span> <span class="token operator">=&gt;</span> el <span class="token operator">===</span> value<span class="token punctuation">)</span>
<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">contains</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token string">'bar'</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token string">'baz'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// =&gt; false</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>另外，Map 和 Set 数据结构有一个<code>has</code>方法，需要注意与<code>includes</code>区分。</p> <ul><li>Map 结构的<code>has</code>方法，是用来查找键名的，比如<code>Map.prototype.has(key)</code>、<code>WeakMap.prototype.has(key)</code>、<code>Reflect.has(target, propertyKey)</code>。</li> <li>Set 结构的<code>has</code>方法，是用来查找值的，比如<code>Set.prototype.has(value)</code>、<code>WeakSet.prototype.has(value)</code>。</li></ul> <h2 id="数组实例的-flat-flatmap"><a href="#数组实例的-flat-flatmap" class="header-anchor">#</a> 数组实例的 flat()，flatMap()</h2> <p>数组的成员有时还是数组，<code>Array.prototype.flat()</code>用于<strong>将嵌套的数组“拉平”，变成一维的数组</strong>。该方法返回一个新数组，对原数据没有影响。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">flat</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token comment">// [1, 2, 3, 4]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，原数组的成员里面有一个数组，<code>flat()</code>方法将子数组的成员取出来，添加在原来的位置。</p> <p><code>flat()</code>默认只会“拉平”一层，如果想要“拉平”多层的嵌套数组，可以将<code>flat()</code>方法的<strong>参数写成一个整数，表示想要拉平的层数，默认为1。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">flat</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token comment">// [1, 2, 3, [4, 5]]</span>

<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">flat</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>
<span class="token comment">// [1, 2, 3, 4, 5]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>flat()</code>的参数为2，表示要“拉平”两层的嵌套数组。</p> <p>如果不管有多少层嵌套，都要转成一维数组，可以用<code>Infinity</code>关键字作为参数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">flat</span><span class="token punctuation">(</span><span class="token number">Infinity</span><span class="token punctuation">)</span>
<span class="token comment">// [1, 2, 3]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果原数组有空位，<code>flat()</code>方法会跳过空位。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">flat</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token comment">// [1, 2, 4, 5]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>flatMap()</code>方法对原数组的每个成员执行一个函数（相当于执行<code>Array.prototype.map()</code>），然后对返回值组成的数组执行<code>flat()</code>方法。该方法返回一个新数组，不改变原数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 相当于 [2, 3, 4].map(x =&gt; [x, x*2]).flat()</span>
<span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">flatMap</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> x <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token comment">// [2, 4, 3, 6, 4, 8]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>flatMap()</code><strong>只能展开一层数组</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 相当于 [[[2]], [[4]], [[6]], [[8]]].flat()</span>
<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">flatMap</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> <span class="token punctuation">[</span><span class="token punctuation">[</span>x <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token comment">// [[2], [4], [6], [8]]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，遍历函数返回的是一个双层的数组，但是默认只能展开一层，因此<code>flatMap()</code>返回的还是一个嵌套数组。</p> <p><code>flatMap()</code>方法的参数是一个遍历函数，该函数可以接受三个参数，分别是当前数组成员、当前数组成员的位置（从零开始）、原数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>arr<span class="token punctuation">.</span><span class="token function">flatMap</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token function">callback</span><span class="token punctuation">(</span><span class="token parameter">currentValue<span class="token punctuation">[</span><span class="token punctuation">,</span> index<span class="token punctuation">[</span><span class="token punctuation">,</span> array<span class="token punctuation">]</span><span class="token punctuation">]</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span><span class="token punctuation">[</span><span class="token punctuation">,</span> thisArg<span class="token punctuation">]</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>flatMap()</code>方法还可以有第二个参数，用来绑定遍历函数里面的<code>this</code>。</p> <h2 id="数组的空位"><a href="#数组的空位" class="header-anchor">#</a> 数组的空位</h2> <p>数组的空位指，数组的某一个位置没有任何值。比如，<code>Array</code>构造函数返回的数组都是空位。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">Array</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// [, , ,]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>Array(3)</code>返回一个具有 3 个空位的数组。</p> <p>注意，空位不是<code>undefined</code>，一个位置的值等于<code>undefined</code>，依然是有值的。空位是没有任何值，<code>in</code>运算符可以说明这一点。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">0</span> <span class="token keyword">in</span> <span class="token punctuation">[</span><span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">]</span> <span class="token comment">// true</span>
<span class="token number">0</span> <span class="token keyword">in</span> <span class="token punctuation">[</span><span class="token punctuation">,</span> <span class="token punctuation">,</span> <span class="token punctuation">,</span><span class="token punctuation">]</span> <span class="token comment">// false</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码说明，第一个数组的 0 号位置是有值的，第二个数组的 0 号位置没有值。</p> <p>ES5 对空位的处理，已经很不一致了，大多数情况下会忽略空位。</p> <ul><li><code>forEach()</code>, <code>filter()</code>, <code>reduce()</code>, <code>every()</code> 和<code>some()</code>都会跳过空位。</li> <li><code>map()</code>会跳过空位，但会保留这个值</li> <li><code>join()</code>和<code>toString()</code>会将空位视为<code>undefined</code>，而<code>undefined</code>和<code>null</code>会被处理成空字符串。</li></ul> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// forEach方法</span>
<span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span>i</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1</span>

<span class="token comment">// filter方法</span>
<span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token punctuation">,</span><span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> <span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// ['a','b']</span>

<span class="token comment">// every方法</span>
<span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">every</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> x<span class="token operator">===</span><span class="token string">'a'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>

<span class="token comment">// reduce方法</span>
<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span>y</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> x<span class="token operator">+</span>y<span class="token punctuation">)</span> <span class="token comment">// 3</span>

<span class="token comment">// some方法</span>
<span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">some</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> x <span class="token operator">!==</span> <span class="token string">'a'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>

<span class="token comment">// map方法</span>
<span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// [,1]</span>

<span class="token comment">// join方法</span>
<span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token keyword">undefined</span><span class="token punctuation">,</span><span class="token keyword">null</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">'#'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;#a##&quot;</span>

<span class="token comment">// toString方法</span>
<span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token keyword">undefined</span><span class="token punctuation">,</span><span class="token keyword">null</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;,a,,&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br></div></div><p>ES6 则是明确将空位转为<code>undefined</code>。</p> <p><code>Array.from</code>方法会将数组的空位，转为<code>undefined</code>，也就是说，这个方法不会忽略空位。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token punctuation">,</span><span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token comment">// [ &quot;a&quot;, undefined, &quot;b&quot; ]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>扩展运算符（<code>...</code>）也会将空位转为<code>undefined</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token punctuation">,</span><span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">]</span>
<span class="token comment">// [ &quot;a&quot;, undefined, &quot;b&quot; ]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>copyWithin()</code>会连空位一起拷贝。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token string">'b'</span><span class="token punctuation">,</span><span class="token punctuation">,</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">copyWithin</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// [,&quot;a&quot;,,&quot;a&quot;]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>fill()</code>会将空位视为正常的数组位置。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">fill</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;,&quot;a&quot;,&quot;a&quot;]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>for...of</code>循环也会遍历空位。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">,</span> <span class="token punctuation">,</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token keyword">of</span> arr<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 1</span>
<span class="token comment">// 1</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，数组<code>arr</code>有两个空位，<code>for...of</code>并没有忽略它们。如果改成<code>map</code>方法遍历，空位是会跳过的。</p> <p><code>entries()</code>、<code>keys()</code>、<code>values()</code>、<code>find()</code>和<code>findIndex()</code>会将空位处理成<code>undefined</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// entries()</span>
<span class="token punctuation">[</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">entries</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token comment">// [[0,undefined], [1,&quot;a&quot;]]</span>

<span class="token comment">// keys()</span>
<span class="token punctuation">[</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token comment">// [0,1]</span>

<span class="token comment">// values()</span>
<span class="token punctuation">[</span><span class="token operator">...</span><span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token comment">// [undefined,&quot;a&quot;]</span>

<span class="token comment">// find()</span>
<span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">find</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> <span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// undefined</span>

<span class="token comment">// findIndex()</span>
<span class="token punctuation">[</span><span class="token punctuation">,</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">findIndex</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> <span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>由于空位的处理规则非常不统一，所以建议避免出现空位。</p> <h2 id="array-prototype-sort-的排序稳定性"><a href="#array-prototype-sort-的排序稳定性" class="header-anchor">#</a> Array.prototype.sort() 的排序稳定性</h2> <p>排序稳定性（stable sorting）是排序算法的重要属性，指的是排序关键字相同的项目，排序前后的顺序不变。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span>
  <span class="token string">'peach'</span><span class="token punctuation">,</span>
  <span class="token string">'straw'</span><span class="token punctuation">,</span>
  <span class="token string">'apple'</span><span class="token punctuation">,</span>
  <span class="token string">'spork'</span>
<span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> <span class="token function-variable function">stableSorting</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">s1<span class="token punctuation">,</span> s2</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>s1<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">&lt;</span> s2<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

arr<span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span>stableSorting<span class="token punctuation">)</span>
<span class="token comment">// [&quot;apple&quot;, &quot;peach&quot;, &quot;straw&quot;, &quot;spork&quot;]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>上面代码对数组<code>arr</code>按照首字母进行排序。排序结果中，<code>straw</code>在<code>spork</code>的前面，跟原始顺序一致，所以排序算法<code>stableSorting</code>是稳定排序。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token function-variable function">unstableSorting</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">s1<span class="token punctuation">,</span> s2</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>s1<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">&lt;=</span> s2<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

arr<span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span>unstableSorting<span class="token punctuation">)</span>
<span class="token comment">// [&quot;apple&quot;, &quot;peach&quot;, &quot;spork&quot;, &quot;straw&quot;]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，排序结果是<code>spork</code>在<code>straw</code>前面，跟原始顺序相反，所以排序算法<code>unstableSorting</code>是不稳定的。</p> <p>常见的排序算法之中，插入排序、合并排序、冒泡排序等都是稳定的，堆排序、快速排序等是不稳定的。不稳定排序的主要缺点是，多重排序时可能会产生问题。假设有一个姓和名的列表，要求按照“姓氏为主要关键字，名字为次要关键字”进行排序。开发者可能会先按名字排序，再按姓氏进行排序。如果排序算法是稳定的，这样就可以达到“先姓氏，后名字”的排序效果。如果是不稳定的，就不行。</p> <p>早先的 ECMAScript 没有规定，<code>Array.prototype.sort()</code>的默认排序算法是否稳定，留给浏览器自己决定，这导致某些实现是不稳定的。<a href="https://github.com/tc39/ecma262/pull/1340" target="_blank" rel="noopener noreferrer">ES2019<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 明确规定，<code>Array.prototype.sort()</code>的默认排序算法必须稳定。这个规定已经做到了，现在 JavaScript 各个主要实现的默认排序算法都是稳定的。</p></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《ES6 教程》笔记/09.数组的扩展.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="tags"><a href="/blog/tags/?tag=ES6" title="标签">#ES6</a></div> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2022/12/14, 19:36:42</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/blog/pages/8ed309d668b20264/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">函数的扩展</div></a> <a href="/blog/pages/b5e3e0a0ff6e9c25/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">对象的扩展</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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