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class="sidebar-link">State&amp;生命周期</a></li><li><a href="/blog/pages/f27775/" class="sidebar-link">事件处理</a></li><li><a href="/blog/pages/8fcda8/" class="sidebar-link">条件渲染</a></li><li><a href="/blog/pages/88f4b0/" aria-current="page" class="active sidebar-link">列表&amp;Key</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/blog/pages/88f4b0/#渲染多个组件" class="sidebar-link">渲染多个组件</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/88f4b0/#基础列表组件" class="sidebar-link">基础列表组件</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/88f4b0/#key" class="sidebar-link">key</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/88f4b0/#用-key-提取组件-在map方法内设置key" class="sidebar-link">用 key 提取组件 (在map方法内设置key)</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/88f4b0/#" class="sidebar-link"></a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/88f4b0/#key-只是在兄弟节点之间必须唯一" class="sidebar-link">key 只是在兄弟节点之间必须唯一</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/88f4b0/#不能用key当props名传给子组件" class="sidebar-link">不能用key当props名传给子组件</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/88f4b0/#在-jsx-中嵌入-map" class="sidebar-link">在 JSX 中嵌入 map()</a></li></ul></li><li><a href="/blog/pages/002db7/" class="sidebar-link">表单</a></li><li><a href="/blog/pages/f0e3d2/" class="sidebar-link">状态提升 (共享状态)</a></li><li><a href="/blog/pages/9ae8e8/" class="sidebar-link">组合vs继承</a></li><li><a href="/blog/pages/c689bf/" class="sidebar-link">React哲学</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>高级指引</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Hook</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>案例演示</span> <span class="arrow right"></span></p> <!----></section></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/react/#《React》笔记" data-v-06970110>《React》笔记</a></li><li data-v-06970110><a href="/blog/note/react/#核心概念" data-v-06970110>核心概念</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="https://github.com/heBody" target="_blank" title="作者" class="beLink" data-v-06970110>heBody</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2021-03-24</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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">列表&amp;Key<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="_07-列表-key"><a href="#_07-列表-key" class="header-anchor">#</a> 07. 列表 &amp; Key</h1> <p>如下代码，我们使用 <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map" target="_blank" rel="noopener noreferrer"><code>map()</code><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> 函数让数组中的每一项变双倍，然后我们得到了一个新的列表 <code>doubled</code> 并打印出来：</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">const</span> numbers <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> doubled <span class="token operator">=</span> numbers<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">number</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> number <span class="token operator">*</span> <span class="token number">2</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>doubled<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>在 React 中，把数组转化为<a href="https://zh-hans.reactjs.org/docs/rendering-elements.html" target="_blank" rel="noopener noreferrer">元素<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>列表的过程是相似的。</p> <h2 id="渲染多个组件"><a href="#渲染多个组件" class="header-anchor">#</a> 渲染多个组件</h2> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">const</span> numbers <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">]</span>
<span class="token comment">// 通过map方法将数值放入li，形成一个li标签组成的数值，在一次性放入ul</span>
<span class="token keyword">const</span> listItems <span class="token operator">=</span> numbers<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">number</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>numbers<span class="token punctuation">}</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>

ReactDOM<span class="token punctuation">.</span><span class="token function">render</span><span class="token punctuation">(</span>
	<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>ul</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>listItems<span class="token punctuation">}</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>ul</span><span class="token punctuation">&gt;</span></span>
  document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'root'</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><h2 id="基础列表组件"><a href="#基础列表组件" class="header-anchor">#</a> 基础列表组件</h2> <p>将列表组合成一个组件：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">NumberList</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
  <span class="token keyword">const</span> numbers <span class="token operator">=</span> props<span class="token punctuation">.</span>numbers
  <span class="token keyword">const</span> listItems <span class="token operator">=</span> numbers<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">number</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token comment">// 记得给列表元素添加key</span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span> <span class="token attr-name">key</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>number<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><span class="token punctuation">&gt;</span></span><span class="token plain-text">
    	</span><span class="token punctuation">{</span>number<span class="token punctuation">}</span><span class="token plain-text">
    </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span>

  <span class="token keyword">return</span> <span class="token punctuation">(</span>
  	<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>ul</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>listItems<span class="token punctuation">}</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>ul</span><span class="token punctuation">&gt;</span></span>
  <span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> numbers <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">]</span>
ReactDOM<span class="token punctuation">.</span><span class="token function">render</span><span class="token punctuation">(</span>
	<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">NumberList</span></span> <span class="token attr-name">numbers</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>numbers<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">,</span>
  document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'root'</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></div></div><h2 id="key"><a href="#key" class="header-anchor">#</a> key</h2> <p><strong>key 帮助 React 识别哪些元素改变了</strong>，比如被添加或删除。因此你应当给数组中的每一个元素赋予一个确定的标识。</p> <p>一个元素的 key 最好是这个元素在列表中拥有的<strong>一个独一无二的字符串</strong>。<strong>通常，我们使用数据中的 id 来作为元素的 key</strong></p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">const</span> todoItems <span class="token operator">=</span> todos<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">todo</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span>
  <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span> <span class="token attr-name">key</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>todo<span class="token punctuation">.</span>id<span class="token punctuation">}</span></span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
       </span><span class="token punctuation">{</span>todo<span class="token punctuation">.</span>text<span class="token punctuation">}</span><span class="token plain-text">
  </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></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>当元素没有确定 id 的时候，<strong>万不得已你可以使用元素索引 index 作为 key</strong>：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">const</span> todoItems <span class="token operator">=</span> todos<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">item<span class="token punctuation">,</span> index</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span>
  <span class="token comment">// 只有item中没有id 才用index代替</span>
  <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span> <span class="token attr-name">key</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>index<span class="token punctuation">}</span></span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
        </span><span class="token punctuation">{</span>item<span class="token punctuation">.</span>text<span class="token punctuation">}</span><span class="token plain-text">
  </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></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></div></div><p><strong>如果列表项目的顺序可能会变化，我们不建议使用索引来用作 key 值，因为这样做会导致性能变差，还可能引起组件状态的问题。</strong></p> <p>如果你选择不指定显式的 key 值，那么 React 将默认使用索引用作为列表项目的 key 值</p> <h2 id="用-key-提取组件-在map方法内设置key"><a href="#用-key-提取组件-在map方法内设置key" class="header-anchor">#</a> 用 key 提取组件 (在map方法内设置key)</h2> <p>元素的 key 只有放在就近的数组上下文中才有意义。</p> <p><strong>例子：不正确的使用 key 的方式</strong></p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">ListItem</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> value <span class="token operator">=</span> props<span class="token punctuation">.</span>value<span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token punctuation">(</span>
    <span class="token comment">// 错误！你不需要在这里指定 key：</span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span> <span class="token attr-name">key</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>value<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><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token plain-text">
    </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">NumberList</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> numbers <span class="token operator">=</span> props<span class="token punctuation">.</span>numbers<span class="token punctuation">;</span>
  <span class="token keyword">const</span> listItems <span class="token operator">=</span> numbers<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">number</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span>
    <span class="token comment">// 错误！元素的 key 应该在这里指定：</span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">ListItem</span></span> <span class="token attr-name">value</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>number<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">return</span> <span class="token punctuation">(</span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>ul</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token punctuation">{</span>listItems<span class="token punctuation">}</span><span class="token plain-text">
    </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>ul</span><span class="token punctuation">&gt;</span></span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> numbers <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
ReactDOM<span class="token punctuation">.</span><span class="token function">render</span><span class="token punctuation">(</span>
  <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">NumberList</span></span> <span class="token attr-name">numbers</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>numbers<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">,</span>
  document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'root'</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><span class="line-number">29</span><br></div></div><p>一个好的经验法则是：<strong>在 <code>map()</code> 方法中的元素需要设置 key 属性</strong>。</p> <h2 id=""><a href="#" class="header-anchor">#</a></h2> <h2 id="key-只是在兄弟节点之间必须唯一"><a href="#key-只是在兄弟节点之间必须唯一" class="header-anchor">#</a> key 只是在兄弟节点之间必须唯一</h2> <p>数组元素中使用的 key 在其兄弟节点之间应该是独一无二的。然而，它们<strong>不需要是全局唯一的</strong>。当我们生成两个不同的数组时，我们可以使用相同的 key 值。</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">Blog</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> sidebar <span class="token operator">=</span> <span class="token punctuation">(</span>    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>ul</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token punctuation">{</span>props<span class="token punctuation">.</span>posts<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">post</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span>
        <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span> <span class="token attr-name">key</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>post<span class="token punctuation">.</span>id<span class="token punctuation">}</span></span><span class="token punctuation">&gt;</span></span><span class="token plain-text">          </span><span class="token punctuation">{</span>post<span class="token punctuation">.</span>title<span class="token punctuation">}</span><span class="token plain-text">
        </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
      <span class="token punctuation">)</span><span class="token punctuation">}</span><span class="token plain-text">
    </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>ul</span><span class="token punctuation">&gt;</span></span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> content <span class="token operator">=</span> props<span class="token punctuation">.</span>posts<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">post</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span>    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>div</span> <span class="token attr-name">key</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>post<span class="token punctuation">.</span>id<span class="token punctuation">}</span></span><span class="token punctuation">&gt;</span></span><span class="token plain-text">      </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>h3</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>post<span class="token punctuation">.</span>title<span class="token punctuation">}</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>h3</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>p</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>post<span class="token punctuation">.</span>content<span class="token punctuation">}</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>p</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
    </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token punctuation">(</span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>div</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token punctuation">{</span>sidebar<span class="token punctuation">}</span><span class="token plain-text">      </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>hr</span> <span class="token punctuation">/&gt;</span></span><span class="token plain-text">
      </span><span class="token punctuation">{</span>content<span class="token punctuation">}</span><span class="token plain-text">    </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> posts <span class="token operator">=</span> <span class="token punctuation">[</span>
  <span class="token punctuation">{</span><span class="token literal-property property">id</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">title</span><span class="token operator">:</span> <span class="token string">'Hello World'</span><span class="token punctuation">,</span> <span class="token literal-property property">content</span><span class="token operator">:</span> <span class="token string">'Welcome to learning React!'</span><span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token punctuation">{</span><span class="token literal-property property">id</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token literal-property property">title</span><span class="token operator">:</span> <span class="token string">'Installation'</span><span class="token punctuation">,</span> <span class="token literal-property property">content</span><span class="token operator">:</span> <span class="token string">'You can install React from npm.'</span><span class="token punctuation">}</span>
<span class="token punctuation">]</span><span class="token punctuation">;</span>
ReactDOM<span class="token punctuation">.</span><span class="token function">render</span><span class="token punctuation">(</span>
  <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">Blog</span></span> <span class="token attr-name">posts</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>posts<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">,</span>
  document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'root'</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></div></div><h3 id="不能用key当props名传给子组件"><a href="#不能用key当props名传给子组件" class="header-anchor">#</a> 不能用<code>key</code>当props名传给子组件</h3> <p>key 会传递信息给 React ，但不会传递给你的组件。如果你的组件中需要使用 <code>key</code> 属性的值，请用其他属性名显式传递这个值：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">const</span> content <span class="token operator">=</span> posts<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">post</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span>
  <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">Post</span></span>
    <span class="token attr-name">key</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>post<span class="token punctuation">.</span>id<span class="token punctuation">}</span></span>
    <span class="token attr-name">id</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>post<span class="token punctuation">.</span>id<span class="token punctuation">}</span></span>
    <span class="token attr-name">title</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>post<span class="token punctuation">.</span>title<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></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></div></div><p>上面例子中，<code>Post</code> 组件可以读出 <code>props.id</code>，但是<strong>不能读出 <code>props.key</code></strong>。</p> <h2 id="在-jsx-中嵌入-map"><a href="#在-jsx-中嵌入-map" class="header-anchor">#</a> 在 JSX 中嵌入 map()</h2> <p>JSX 允许在大括号中<a href="https://zh-hans.reactjs.org/docs/introducing-jsx.html#embedding-expressions-in-jsx" target="_blank" rel="noopener noreferrer">嵌入任何表达式<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>，所以我们可以内联 <code>map()</code> 返回的结果：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">NumberList</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> numbers <span class="token operator">=</span> props<span class="token punctuation">.</span>numbers<span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token punctuation">(</span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>ul</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token punctuation">{</span>
        numbers<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">number</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span>
            <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">ListItem</span></span> <span class="token attr-name">key</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>number<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> <span class="token attr-name">value</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>number<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span>
        <span class="token punctuation">)</span>
      <span class="token punctuation">}</span><span class="token plain-text">
    </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>ul</span><span class="token punctuation">&gt;</span></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>这么做有时可以使你的代码更清晰，但有时这种风格也会被滥用。</p> <p>就像在 JavaScript 中一样，何时需要为了可读性提取出一个变量，这完全取决于你。但请记住，如果一个 <code>map()</code> 嵌套了太多层级，那可能就是你<a href="https://zh-hans.reactjs.org/docs/components-and-props.html#extracting-components" target="_blank" rel="noopener noreferrer">提取组件<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>的一个好时机。</p></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《React》笔记/01.核心概念/07.列表&amp;Key.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=React" title="标签">#React</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/8fcda8/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">条件渲染</div></a> <a href="/blog/pages/002db7/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">表单</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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