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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-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06970110>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="javascript:;" data-v-06970110>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>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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">Class 的继承<!----></h1> <div class="theme-vdoing-content content__default"><h1 id="class-的继承"><a href="#class-的继承" class="header-anchor">#</a> Class 的继承</h1> <h2 id="简介"><a href="#简介" class="header-anchor">#</a> 简介</h2> <p>Class 可以通过<code>extends</code>关键字实现继承,这比 ES5 的通过修改原型链实现继承,要清晰和方便很多。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</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></div></div> <p>上面代码定义了一个<code>ColorPoint</code>类,该类通过<code>extends</code>关键字,继承了<code>Point</code>类的所有属性和方法。但是由于没有部署任何代码,所以这两个类完全一样,等于复制了一个<code>Point</code>类。下面,我们在<code>ColorPoint</code>内部加上代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> color</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 调用父类的constructor(x, y)</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>color <span class="token operator">=</span> color<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 punctuation">{</span>
- <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>color <span class="token operator">+</span> <span class="token string">' '</span> <span class="token operator">+</span> <span class="token keyword">super</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 punctuation">;</span> <span class="token comment">// 调用父类的toString()</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><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中,<code>constructor</code>方法和<code>toString</code>方法之中,都出现了<code>super</code>关键字,它在这里表示父类的构造函数,用来新建父类的<code>this</code>对象。</p> <p>**子类必须在<code>constructor</code>方法中调用<code>super</code>方法,否则新建实例时会报错。**这是因为子类自己的<code>this</code>对象,必须先通过父类的构造函数完成塑造,得到与父类同样的实例属性和方法,然后再对其进行加工,加上子类自己的实例属性和方法。<strong>如果不调用<code>super</code>方法,子类就得不到<code>this</code>对象。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span> <span class="token comment">/* ... */</span> <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</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> cp <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ColorPoint</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError</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>ColorPoint</code>继承了父类<code>Point</code>,但是它的构造函数没有调用<code>super</code>方法,导致新建实例时报错。</p> <p>ES5 的继承,实质是先创造子类的实例对象<code>this</code>,然后再将父类的方法添加到<code>this</code>上面(<code>Parent.apply(this)</code>)。ES6 的继承机制完全不同,实质是先将父类实例对象的属性和方法,加到<code>this</code>上面(所以必须先调用<code>super</code>方法),然后再用子类的构造函数修改<code>this</code>。</p> <p>如果子类没有定义<code>constructor</code>方法,这个方法会被默认添加,代码如下。也就是说,不管有没有显式定义,任何一个子类都有<code>constructor</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
- <span class="token punctuation">}</span>
- <span class="token comment">// 等同于</span>
- <span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token operator">...</span>args<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><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>另一个需要注意的地方是,**在子类的构造函数中,只有调用<code>super</code>之后,才可以使用<code>this</code>关键字,否则会报错。**这是因为子类实例的构建,基于父类实例,只有<code>super</code>方法才能调用父类实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</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">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> x<span class="token punctuation">;</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>y <span class="token operator">=</span> y<span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">ColorPoint</span> <span class="token keyword">extends</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> color</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>color <span class="token operator">=</span> color<span class="token punctuation">;</span> <span class="token comment">// ReferenceError</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>color <span class="token operator">=</span> color<span class="token punctuation">;</span> <span class="token comment">// 正确</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><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>上面代码中,子类的<code>constructor</code>方法没有调用<code>super</code>之前,就使用<code>this</code>关键字,结果报错,而放在<code>super</code>方法之后就是正确的。</p> <p>下面是生成子类实例的代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> cp <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ColorPoint</span><span class="token punctuation">(</span><span class="token number">25</span><span class="token punctuation">,</span> <span class="token number">8</span><span class="token punctuation">,</span> <span class="token string">'green'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- cp <span class="token keyword">instanceof</span> <span class="token class-name">ColorPoint</span> <span class="token comment">// true</span>
- cp <span class="token keyword">instanceof</span> <span class="token class-name">Point</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></div></div><p>上面代码中,实例对象<code>cp</code>同时是<code>ColorPoint</code>和<code>Point</code>两个类的实例,这与 ES5 的行为完全一致。</p> <p>最后,父类的静态方法,也会被子类继承。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token keyword">static</span> <span class="token function">hello</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 string">'hello world'</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">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token punctuation">}</span>
- <span class="token constant">B</span><span class="token punctuation">.</span><span class="token function">hello</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// hello world</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>hello()</code>是<code>A</code>类的静态方法,<code>B</code>继承<code>A</code>,也继承了<code>A</code>的静态方法。</p> <h2 id="object-getprototypeof"><a href="#object-getprototypeof" class="header-anchor">#</a> Object.getPrototypeOf()</h2> <p><code>Object.getPrototypeOf</code>方法可以用来从子类上获取父类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>ColorPoint<span class="token punctuation">)</span> <span class="token operator">===</span> Point
- <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>因此,可以使用这个方法判断,一个类是否继承了另一个类。</p> <h2 id="super-关键字"><a href="#super-关键字" class="header-anchor">#</a> super 关键字</h2> <p><strong><code>super</code>这个关键字,既可以当作函数使用,也可以当作对象使用</strong>。在这两种情况下,它的用法完全不同。</p> <p>第一种情况,<code>super</code>作为函数调用时,代表父类的构造函数。ES6 要求,子类的构造函数必须执行一次<code>super</code>函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</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>
- </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,子类<code>B</code>的构造函数之中的<code>super()</code>,代表调用父类的构造函数。这是必须的,否则 JavaScript 引擎会报错。</p> <p>注意,<strong><code>super</code>虽然代表了父类<code>A</code>的构造函数,但是返回的是子类<code>B</code>的实例</strong>,即<code>super</code>内部的<code>this</code>指的是<code>B</code>的实例,因此<code>super()</code>在这里相当于<code>A.prototype.constructor.call(this)</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</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 keyword">new</span><span class="token punctuation">.</span>target<span class="token punctuation">.</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</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">new</span> <span class="token class-name">A</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// A</span>
- <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 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><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>上面代码中,<code>new.target</code>指向当前正在执行的函数。可以看到,在<code>super()</code>执行时,它指向的是子类<code>B</code>的构造函数,而不是父类<code>A</code>的构造函数。也就是说,<code>super()</code>内部的<code>this</code>指向的是<code>B</code>。</p> <p><strong>作为函数时,<code>super()</code>只能用在子类的构造函数之中,用在其他地方就会报错。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 报错</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>上面代码中,<code>super()</code>用在<code>B</code>类的<code>m</code>方法之中,就会造成语法错误。</p> <p>第二种情况,<strong><code>super</code>作为对象时,在普通方法中,指向父类的原型对象;在静态方法中,指向父类。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">p</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 number">2</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</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 keyword">super</span><span class="token punctuation">.</span><span class="token function">p</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">// 2</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">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><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>上面代码中,子类<code>B</code>当中的<code>super.p()</code>,就是将<code>super</code>当作一个对象使用。这时,<code>super</code>在普通方法之中,指向<code>A.prototype</code>,所以<code>super.p()</code>就相当于<code>A.prototype.p()</code>。</p> <p>这里需要注意,由于<code>super</code>指向父类的原型对象,所以定义在父类实例上的方法或属性,是无法通过<code>super</code>调用的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span> <span class="token comment">// 这是定于在实例上的属性</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token keyword">get</span> <span class="token function">m</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 keyword">super</span><span class="token punctuation">.</span>p<span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- b<span class="token punctuation">.</span>m <span class="token comment">// undefined</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>上面代码中,<code>p</code>是父类<code>A</code>实例的属性,<code>super.p</code>就引用不到它。</p> <p>如果属性定义在父类的原型对象上,<code>super</code>就可以取到。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
- <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<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 keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</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 keyword">super</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span> <span class="token comment">// 2</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">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><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,属性<code>x</code>是定义在<code>A.prototype</code>上面的,所以<code>super.x</code>可以取到它的值。</p> <p>ES6 规定,<strong>在子类普通方法中通过<code>super</code>调用父类的方法时,方法内部的<code>this</code>指向当前的子类实例</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token function">print</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 keyword">this</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token keyword">this</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 function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">print</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> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- b<span class="token punctuation">.</span><span class="token function">m</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><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><p>上面代码中,<code>super.print()</code>虽然调用的是<code>A.prototype.print()</code>,但是<code>A.prototype.print()</code>内部的<code>this</code>指向子类<code>B</code>的实例,导致输出的是<code>2</code>,而不是<code>1</code>。也就是说,实际上执行的是<code>super.print.call(this)</code>。</p> <p>由于<code>this</code>指向子类实例,所以如果通过<code>super</code>对某个属性赋值,这时<code>super</code>就是<code>this</code>,赋值的属性会变成子类实例的属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>x <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">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token keyword">this</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 keyword">super</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">3</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 keyword">super</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
- console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">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><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>上面代码中,<code>super.x</code>赋值为<code>3</code>,这时等同于对<code>this.x</code>赋值为<code>3</code>。而当读取<code>super.x</code>的时候,读的是<code>A.prototype.x</code>,所以返回<code>undefined</code>。</p> <p><strong>如果<code>super</code>作为对象,用在静态方法之中,这时<code>super</code>将指向父类,而不是父类的原型对象。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">Parent</span> <span class="token punctuation">{</span>
- <span class="token keyword">static</span> <span class="token function">myMethod</span><span class="token punctuation">(</span><span class="token parameter">msg</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 string">'static'</span><span class="token punctuation">,</span> msg<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token function">myMethod</span><span class="token punctuation">(</span><span class="token parameter">msg</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 string">'instance'</span><span class="token punctuation">,</span> msg<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">Child</span> <span class="token keyword">extends</span> <span class="token class-name">Parent</span> <span class="token punctuation">{</span>
- <span class="token keyword">static</span> <span class="token function">myMethod</span><span class="token punctuation">(</span><span class="token parameter">msg</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">myMethod</span><span class="token punctuation">(</span>msg<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token function">myMethod</span><span class="token punctuation">(</span><span class="token parameter">msg</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">myMethod</span><span class="token punctuation">(</span>msg<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- Child<span class="token punctuation">.</span><span class="token function">myMethod</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 comment">// static 1</span>
- <span class="token keyword">var</span> child <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Child</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- child<span class="token punctuation">.</span><span class="token function">myMethod</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">// instance 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><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><span class="line-number">24</span><br></div></div><p>上面代码中,<code>super</code>在静态方法之中指向父类,在普通方法之中指向父类的原型对象。</p> <p>另外,<strong>在子类的静态方法中通过<code>super</code>调用父类的方法时,方法内部的<code>this</code>指向当前的子类,而不是子类的实例。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">static</span> <span class="token function">print</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 keyword">this</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token keyword">this</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 keyword">static</span> <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">print</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 constant">B</span><span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
- <span class="token constant">B</span><span class="token punctuation">.</span><span class="token function">m</span><span class="token punctuation">(</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><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><p>上面代码中,静态方法<code>B.m</code>里面,<code>super.print</code>指向父类的静态方法。这个方法里面的<code>this</code>指向的是<code>B</code>,而不是<code>B</code>的实例。</p> <p>注意,使用<code>super</code>的时候,必须显式指定是作为函数、还是作为对象使用,否则会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</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 keyword">super</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 报错</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,<code>console.log(super)</code>当中的<code>super</code>,无法看出是作为函数使用,还是作为对象使用,所以 JavaScript 引擎解析代码的时候就会报错。这时,如果能清晰地表明<code>super</code>的数据类型,就不会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</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 keyword">super</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 keyword">instanceof</span> <span class="token class-name">B</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">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><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>super.valueOf()</code>表明<code>super</code>是一个对象,因此就不会报错。同时,由于<code>super</code>使得<code>this</code>指向<code>B</code>的实例,所以<code>super.valueOf()</code>返回的是一个<code>B</code>的实例。</p> <p>最后,由于对象总是继承其他对象的,所以可以在任意一个对象中,使用<code>super</code>关键字。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> obj <span class="token operator">=</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 punctuation">{</span>
- <span class="token keyword">return</span> <span class="token string">"MyObject: "</span> <span class="token operator">+</span> <span class="token keyword">super</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 punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span><span class="token punctuation">;</span>
- obj<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// MyObject: [object Object]</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><h2 id="类的-prototype-属性和-proto-属性"><a href="#类的-prototype-属性和-proto-属性" class="header-anchor">#</a> 类的 prototype 属性和__proto__属性</h2> <p>大多数浏览器的 ES5 实现之中,每一个对象都有<code>__proto__</code>属性,指向对应的构造函数的<code>prototype</code>属性。Class 作为构造函数的语法糖,同时有<code>prototype</code>属性和<code>__proto__</code>属性,因此同时存在两条继承链。</p> <p>(1)子类的<code>__proto__</code>属性,表示构造函数的继承,总是指向父类。</p> <p>(2)子类<code>prototype</code>属性的<code>__proto__</code>属性,表示方法的继承,总是指向父类的<code>prototype</code>属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token punctuation">}</span>
- <span class="token constant">B</span><span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> <span class="token constant">A</span> <span class="token comment">// true</span>
- <span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> <span class="token class-name">A</span><span class="token punctuation">.</span>prototype <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,子类<code>B</code>的<code>__proto__</code>属性指向父类<code>A</code>,子类<code>B</code>的<code>prototype</code>属性的<code>__proto__</code>属性指向父类<code>A</code>的<code>prototype</code>属性。</p> <p>这样的结果是因为,类的继承是按照下面的模式实现的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token punctuation">{</span>
- <span class="token punctuation">}</span>
- <span class="token comment">// B 的实例继承 A 的实例</span>
- Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token comment">// B 继承 A 的静态属性</span>
- Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token constant">B</span><span class="token punctuation">,</span> <span class="token constant">A</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">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><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>《对象的扩展》一章给出过<code>Object.setPrototypeOf</code>方法的实现。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function-variable function">setPrototypeOf</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> proto</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- obj<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> proto<span class="token punctuation">;</span>
- <span class="token keyword">return</span> obj<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>因此,就得到了上面的结果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token comment">// 等同于</span>
- <span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">;</span>
- Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token constant">B</span><span class="token punctuation">,</span> <span class="token constant">A</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token comment">// 等同于</span>
- <span class="token constant">B</span><span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> <span class="token constant">A</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>这两条继承链,可以这样理解:作为一个对象,子类(<code>B</code>)的原型(<code>__proto__</code>属性)是父类(<code>A</code>);作为一个构造函数,子类(<code>B</code>)的原型对象(<code>prototype</code>属性)是父类的原型对象(<code>prototype</code>属性)的实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token class-name">B</span><span class="token punctuation">.</span>prototype <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span><span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token comment">// 等同于</span>
- <span class="token class-name">B</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>__proto__ <span class="token operator">=</span> <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<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>extends</code>关键字后面可以跟多种类型的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</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></div></div><p>上面代码的<code>A</code>,只要是一个有<code>prototype</code>属性的函数,就能被<code>B</code>继承。由于函数都有<code>prototype</code>属性(除了<code>Function.prototype</code>函数),因此<code>A</code>可以是任意函数。</p> <p>下面,讨论两种情况。第一种,子类继承<code>Object</code>类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token keyword">extends</span> <span class="token class-name">Object</span> <span class="token punctuation">{</span>
- <span class="token punctuation">}</span>
- <span class="token constant">A</span><span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> Object <span class="token comment">// true</span>
- <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype <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></div></div><p>这种情况下,<code>A</code>其实就是构造函数<code>Object</code>的复制,<code>A</code>的实例就是<code>Object</code>的实例。</p> <p>第二种情况,不存在任何继承。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
- <span class="token punctuation">}</span>
- <span class="token constant">A</span><span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> <span class="token class-name">Function</span><span class="token punctuation">.</span>prototype <span class="token comment">// true</span>
- <span class="token class-name">A</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype <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></div></div><p>这种情况下,<code>A</code>作为一个基类(即不存在任何继承),就是一个普通函数,所以直接继承<code>Function.prototype</code>。但是,<code>A</code>调用后返回一个空对象(即<code>Object</code>实例),所以<code>A.prototype.__proto__</code>指向构造函数(<code>Object</code>)的<code>prototype</code>属性。</p> <h3 id="实例的-proto-属性"><a href="#实例的-proto-属性" class="header-anchor">#</a> 实例的 __proto__ 属性</h3> <p>子类实例的<code>__proto__</code>属性的<code>__proto__</code>属性,指向父类实例的<code>__proto__</code>属性。也就是说,子类的原型的原型,是父类的原型。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> p1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Point</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 keyword">var</span> p2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ColorPoint</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 string">'red'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- p2<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> p1<span class="token punctuation">.</span>__proto__ <span class="token comment">// false</span>
- p2<span class="token punctuation">.</span>__proto__<span class="token punctuation">.</span>__proto__ <span class="token operator">===</span> p1<span class="token punctuation">.</span>__proto__ <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></div></div><p>上面代码中,<code>ColorPoint</code>继承了<code>Point</code>,导致前者原型的原型是后者的原型。</p> <p>因此,通过子类实例的<code>__proto__.__proto__</code>属性,可以修改父类实例的行为。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>p2<span class="token punctuation">.</span>__proto__<span class="token punctuation">.</span>__proto__<span class="token punctuation">.</span><span class="token function-variable function">printName</span> <span class="token operator">=</span> <span class="token keyword">function</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 string">'Ha'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span><span class="token punctuation">;</span>
- p1<span class="token punctuation">.</span><span class="token function">printName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// "Ha"</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>上面代码在<code>ColorPoint</code>的实例<code>p2</code>上向<code>Point</code>类添加方法,结果影响到了<code>Point</code>的实例<code>p1</code>。</p> <h2 id="原生构造函数的继承"><a href="#原生构造函数的继承" class="header-anchor">#</a> 原生构造函数的继承</h2> <p>原生构造函数是指语言内置的构造函数,通常用来生成数据结构。ECMAScript 的原生构造函数大致有下面这些。</p> <ul><li>Boolean()</li> <li>Number()</li> <li>String()</li> <li>Array()</li> <li>Date()</li> <li>Function()</li> <li>RegExp()</li> <li>Error()</li> <li>Object()</li></ul> <p>以前,这些原生构造函数是无法继承的,比如,不能自己定义一个<code>Array</code>的子类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">MyArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token function">Array</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> arguments<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token class-name">MyArray</span><span class="token punctuation">.</span>prototype <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</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 punctuation">{</span>
- <span class="token literal-property property">constructor</span><span class="token operator">:</span> <span class="token punctuation">{</span>
- <span class="token literal-property property">value</span><span class="token operator">:</span> MyArray<span class="token punctuation">,</span>
- <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
- <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
- <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">true</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><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>上面代码定义了一个继承 Array 的<code>MyArray</code>类。但是,这个类的行为与<code>Array</code>完全不一致。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> colors <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- colors<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token string">"red"</span><span class="token punctuation">;</span>
- colors<span class="token punctuation">.</span>length <span class="token comment">// 0</span>
- colors<span class="token punctuation">.</span>length <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
- colors<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token comment">// "red"</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>之所以会发生这种情况,是因为子类无法获得原生构造函数的内部属性,通过<code>Array.apply()</code>或者分配给原型对象都不行。原生构造函数会忽略<code>apply</code>方法传入的<code>this</code>,也就是说,原生构造函数的<code>this</code>无法绑定,导致拿不到内部属性。</p> <p>ES5 是先新建子类的实例对象<code>this</code>,再将父类的属性添加到子类上,由于父类的内部属性无法获取,导致无法继承原生的构造函数。比如,<code>Array</code>构造函数有一个内部属性<code>[[DefineOwnProperty]]</code>,用来定义新属性时,更新<code>length</code>属性,这个内部属性无法在子类获取,导致子类的<code>length</code>属性行为不正常。</p> <p>下面的例子中,我们想让一个普通对象继承<code>Error</code>对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> e <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
- Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span><span class="token function">Error</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">)</span>
- <span class="token comment">// [ 'stack' ]</span>
- Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span>
- <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></div></div><p>上面代码中,我们想通过<code>Error.call(e)</code>这种写法,让普通对象<code>e</code>具有<code>Error</code>对象的实例属性。但是,<code>Error.call()</code>完全忽略传入的第一个参数,而是返回一个新对象,<code>e</code>本身没有任何变化。这证明了<code>Error.call(e)</code>这种写法,无法继承原生构造函数。</p> <p>ES6 允许继承原生构造函数定义子类,因为 ES6 是先新建父类的实例对象<code>this</code>,然后再用子类的构造函数修饰<code>this</code>,使得父类的所有行为都可以继承。下面是一个继承<code>Array</code>的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">MyArray</span> <span class="token keyword">extends</span> <span class="token class-name">Array</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token operator">...</span>args<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyArray</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 operator">=</span> <span class="token number">12</span><span class="token punctuation">;</span>
- arr<span class="token punctuation">.</span>length <span class="token comment">// 1</span>
- arr<span class="token punctuation">.</span>length <span class="token operator">=</span> <span class="token number">0</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 comment">// undefined</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>上面代码定义了一个<code>MyArray</code>类,继承了<code>Array</code>构造函数,因此就可以从<code>MyArray</code>生成数组的实例。这意味着,ES6 可以自定义原生数据结构(比如<code>Array</code>、<code>String</code>等)的子类,这是 ES5 无法做到的。</p> <p>上面这个例子也说明,<code>extends</code>关键字不仅可以用来继承类,还可以用来继承原生的构造函数。因此可以在原生数据结构的基础上,定义自己的数据结构。下面就是定义了一个带版本功能的数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">VersionedArray</span> <span class="token keyword">extends</span> <span class="token class-name">Array</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>history <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 punctuation">}</span>
- <span class="token function">commit</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>history<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">slice</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 function">revert</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">splice</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">.</span>length<span class="token punctuation">,</span> <span class="token operator">...</span><span class="token keyword">this</span><span class="token punctuation">.</span>history<span class="token punctuation">[</span><span class="token keyword">this</span><span class="token punctuation">.</span>history<span class="token punctuation">.</span>length <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 punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">var</span> x <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">VersionedArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- x<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- x<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- x <span class="token comment">// [1, 2]</span>
- x<span class="token punctuation">.</span>history <span class="token comment">// [[]]</span>
- x<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- x<span class="token punctuation">.</span>history <span class="token comment">// [[], [1, 2]]</span>
- x<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- x <span class="token comment">// [1, 2, 3]</span>
- x<span class="token punctuation">.</span>history <span class="token comment">// [[], [1, 2]]</span>
- x<span class="token punctuation">.</span><span class="token function">revert</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- x <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><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><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br></div></div><p>上面代码中,<code>VersionedArray</code>会通过<code>commit</code>方法,将自己的当前状态生成一个版本快照,存入<code>history</code>属性。<code>revert</code>方法用来将数组重置为最新一次保存的版本。除此之外,<code>VersionedArray</code>依然是一个普通数组,所有原生的数组方法都可以在它上面调用。</p> <p>下面是一个自定义<code>Error</code>子类的例子,可以用来定制报错时的行为。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">ExtendableError</span> <span class="token keyword">extends</span> <span class="token class-name">Error</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">message</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>message <span class="token operator">=</span> message<span class="token punctuation">;</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>stack <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span>stack<span class="token punctuation">;</span>
- <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>constructor<span class="token punctuation">.</span>name<span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">class</span> <span class="token class-name">MyError</span> <span class="token keyword">extends</span> <span class="token class-name">ExtendableError</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">m</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">super</span><span class="token punctuation">(</span>m<span class="token punctuation">)</span><span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">var</span> myerror <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyError</span><span class="token punctuation">(</span><span class="token string">'ll'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- myerror<span class="token punctuation">.</span>message <span class="token comment">// "ll"</span>
- myerror <span class="token keyword">instanceof</span> <span class="token class-name">Error</span> <span class="token comment">// true</span>
- myerror<span class="token punctuation">.</span>name <span class="token comment">// "MyError"</span>
- myerror<span class="token punctuation">.</span>stack
- <span class="token comment">// Error</span>
- <span class="token comment">// at MyError.ExtendableError</span>
- <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><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>注意,继承<code>Object</code>的子类,有一个<a href="http://stackoverflow.com/questions/36203614/super-does-not-pass-arguments-when-instantiating-a-class-extended-from-object" target="_blank" rel="noopener noreferrer">行为差异<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>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">NewObj</span> <span class="token keyword">extends</span> <span class="token class-name">Object</span><span class="token punctuation">{</span>
- <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
- <span class="token keyword">super</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 punctuation">}</span>
- <span class="token keyword">var</span> o <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">NewObj</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">attr</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
- o<span class="token punctuation">.</span>attr <span class="token operator">===</span> <span class="token boolean">true</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><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,<code>NewObj</code>继承了<code>Object</code>,但是无法通过<code>super</code>方法向父类<code>Object</code>传参。这是因为 ES6 改变了<code>Object</code>构造函数的行为,一旦发现<code>Object</code>方法不是通过<code>new Object()</code>这种形式调用,ES6 规定<code>Object</code>构造函数会忽略参数。</p> <h2 id="mixin-模式的实现"><a href="#mixin-模式的实现" class="header-anchor">#</a> Mixin 模式的实现</h2> <p>Mixin 指的是多个对象合成一个新的对象,新对象具有各个组成成员的接口。它的最简单实现如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> a <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 string">'a'</span>
- <span class="token punctuation">}</span><span class="token punctuation">;</span>
- <span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token punctuation">{</span>
- <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token string">'b'</span>
- <span class="token punctuation">}</span><span class="token punctuation">;</span>
- <span class="token keyword">const</span> c <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token operator">...</span>a<span class="token punctuation">,</span> <span class="token operator">...</span>b<span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// {a: 'a', b: '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><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中,<code>c</code>对象是<code>a</code>对象和<code>b</code>对象的合成,具有两者的接口。</p> <p>下面是一个更完备的实现,将多个类的接口“混入”(mix in)另一个类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">mix</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>mixins</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">class</span> <span class="token class-name">Mix</span> <span class="token punctuation">{</span>
- <span class="token function">constructor</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> mixin <span class="token keyword">of</span> mixins<span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token function">copyProperties</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> <span class="token keyword">new</span> <span class="token class-name">mixin</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">// 拷贝实例属性</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> mixin <span class="token keyword">of</span> mixins<span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token function">copyProperties</span><span class="token punctuation">(</span>Mix<span class="token punctuation">,</span> mixin<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 拷贝静态属性</span>
- <span class="token function">copyProperties</span><span class="token punctuation">(</span><span class="token class-name">Mix</span><span class="token punctuation">.</span>prototype<span class="token punctuation">,</span> mixin<span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 拷贝原型属性</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">return</span> Mix<span class="token punctuation">;</span>
- <span class="token punctuation">}</span>
- <span class="token keyword">function</span> <span class="token function">copyProperties</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> source</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> key <span class="token keyword">of</span> Reflect<span class="token punctuation">.</span><span class="token function">ownKeys</span><span class="token punctuation">(</span>source<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">if</span> <span class="token punctuation">(</span> key <span class="token operator">!==</span> <span class="token string">'constructor'</span>
- <span class="token operator">&&</span> key <span class="token operator">!==</span> <span class="token string">'prototype'</span>
- <span class="token operator">&&</span> key <span class="token operator">!==</span> <span class="token string">'name'</span>
- <span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token keyword">let</span> desc <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>source<span class="token punctuation">,</span> key<span class="token punctuation">)</span><span class="token punctuation">;</span>
- Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> desc<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><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><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br></div></div><p>上面代码的<code>mix</code>函数,可以将多个对象合成为一个类。使用的时候,只要继承这个类即可。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">DistributedEdit</span> <span class="token keyword">extends</span> <span class="token class-name">mix</span><span class="token punctuation">(</span>Loggable<span class="token punctuation">,</span> Serializable<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></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/22.Class 的继承.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/e831e1593c82bbe0/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">Class 的基本语法</div></a> <a href="/blog/pages/efe2fb04eb8ac5fb/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">Module 的语法</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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