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  19.   <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></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端" class="dropdown-title"><a href="/blog/web/" class="link-title">前端</a> <span class="title" style="display:none;">前端</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>前端文章</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/pages/8143cc480faf9a11/" class="nav-link">JavaScript</a></li></ul></li><li class="dropdown-item"><h4>学习笔记</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/note/javascript/" class="nav-link">《JavaScript教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/js/" class="nav-link">《JavaScript高级程序设计》</a></li><li class="dropdown-subitem"><a href="/blog/note/es6/" class="nav-link">《ES6 教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/vue/" class="nav-link">《Vue》</a></li><li class="dropdown-subitem"><a href="/blog/note/react/" class="nav-link">《React》</a></li><li class="dropdown-subitem"><a href="/blog/note/typescript-axios/" class="nav-link">《TypeScript 从零实现 axios》</a></li><li class="dropdown-subitem"><a href="/blog/note/git/" class="nav-link">《Git》</a></li><li class="dropdown-subitem"><a href="/blog/pages/51afd6/" class="nav-link">TypeScript</a></li><li class="dropdown-subitem"><a href="/blog/pages/4643cd/" class="nav-link">JS设计模式总结</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="页面" class="dropdown-title"><a href="/blog/ui/" class="link-title">页面</a> <span class="title" style="display:none;">页面</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/8309a5b876fc95e3/" class="nav-link">HTML</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/0a83b083bdf257cb/" class="nav-link">CSS</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="技术" class="dropdown-title"><a href="/blog/technology/" class="link-title">技术</a> <span class="title" style="display:none;">技术</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/9a7ee40fc232253e/" class="nav-link">技术文档</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/4c778760be26d8b3/" class="nav-link">GitHub技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/117708e0af7f0bd9/" class="nav-link">Nodejs</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/41f87d890d0a02af/" class="nav-link">博客搭建</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="更多" class="dropdown-title"><a href="/blog/more/" class="link-title">更多</a> <span class="title" style="display:none;">更多</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/f2a556/" class="nav-link">学习</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/aea6571b7a8bae86/" class="nav-link">面试</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/2d615df9a36a98ed/" class="nav-link">心情杂货</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/baaa02/" class="nav-link">实用技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/friends/" class="nav-link">友情链接</a></li></ul></div></div><div class="nav-item"><a href="/blog/about/" class="nav-link">关于</a></div><div class="nav-item"><a href="/blog/pages/beb6c0bd8a66cea6/" class="nav-link">收藏</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="索引" class="dropdown-title"><a href="/blog/archives/" class="link-title">索引</a> <span class="title" style="display:none;">索引</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/categories/" class="nav-link">分类</a></li><li class="dropdown-item"><!----> <a href="/blog/tags/" class="nav-link">标签</a></li><li class="dropdown-item"><!----> <a href="/blog/archives/" class="nav-link">归档</a></li></ul></div></div> <a href="https://github.com/heBody/blog" target="_blank" rel="noopener noreferrer" class="repo-link">
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  23.   <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></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端" class="dropdown-title"><a href="/blog/web/" class="link-title">前端</a> <span class="title" style="display:none;">前端</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>前端文章</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/pages/8143cc480faf9a11/" class="nav-link">JavaScript</a></li></ul></li><li class="dropdown-item"><h4>学习笔记</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/note/javascript/" class="nav-link">《JavaScript教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/js/" class="nav-link">《JavaScript高级程序设计》</a></li><li class="dropdown-subitem"><a href="/blog/note/es6/" class="nav-link">《ES6 教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/vue/" class="nav-link">《Vue》</a></li><li class="dropdown-subitem"><a href="/blog/note/react/" class="nav-link">《React》</a></li><li class="dropdown-subitem"><a href="/blog/note/typescript-axios/" class="nav-link">《TypeScript 从零实现 axios》</a></li><li class="dropdown-subitem"><a href="/blog/note/git/" class="nav-link">《Git》</a></li><li class="dropdown-subitem"><a href="/blog/pages/51afd6/" class="nav-link">TypeScript</a></li><li class="dropdown-subitem"><a href="/blog/pages/4643cd/" class="nav-link">JS设计模式总结</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="页面" class="dropdown-title"><a href="/blog/ui/" class="link-title">页面</a> <span class="title" style="display:none;">页面</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/8309a5b876fc95e3/" class="nav-link">HTML</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/0a83b083bdf257cb/" class="nav-link">CSS</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="技术" class="dropdown-title"><a href="/blog/technology/" class="link-title">技术</a> <span class="title" style="display:none;">技术</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/9a7ee40fc232253e/" class="nav-link">技术文档</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/4c778760be26d8b3/" class="nav-link">GitHub技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/117708e0af7f0bd9/" class="nav-link">Nodejs</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/41f87d890d0a02af/" class="nav-link">博客搭建</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="更多" class="dropdown-title"><a href="/blog/more/" class="link-title">更多</a> <span class="title" style="display:none;">更多</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/f2a556/" class="nav-link">学习</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/aea6571b7a8bae86/" class="nav-link">面试</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/2d615df9a36a98ed/" class="nav-link">心情杂货</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/baaa02/" class="nav-link">实用技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/friends/" class="nav-link">友情链接</a></li></ul></div></div><div class="nav-item"><a href="/blog/about/" class="nav-link">关于</a></div><div class="nav-item"><a href="/blog/pages/beb6c0bd8a66cea6/" class="nav-link">收藏</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="索引" class="dropdown-title"><a href="/blog/archives/" class="link-title">索引</a> <span class="title" style="display:none;">索引</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/categories/" class="nav-link">分类</a></li><li class="dropdown-item"><!----> <a href="/blog/tags/" class="nav-link">标签</a></li><li class="dropdown-item"><!----> <a href="/blog/archives/" class="nav-link">归档</a></li></ul></div></div> <a href="https://github.com/heBody/blog" target="_blank" rel="noopener noreferrer" class="repo-link">
    24. 

  24.     GitHub
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  25.     <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></nav>  <ul class="sidebar-links"><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading open"><span>核心概念</span> <span class="arrow down"></span></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/blog/pages/5df969/" class="sidebar-link">JSX简介</a></li><li><a href="/blog/pages/b5d372/" class="sidebar-link">元素渲染</a></li><li><a href="/blog/pages/9ac43a/" class="sidebar-link">组件&amp;Props</a></li><li><a href="/blog/pages/1e3ca2/" 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/" aria-current="page" class="active sidebar-link">条件渲染</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/blog/pages/8fcda8/#元素变量" class="sidebar-link">元素变量</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/8fcda8/#与运算符" class="sidebar-link">与运算符 &amp;&amp;</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/8fcda8/#三目运算符" class="sidebar-link">三目运算符</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/8fcda8/#阻止组件渲染" class="sidebar-link">阻止组件渲染</a></li></ul></li><li><a href="/blog/pages/88f4b0/" class="sidebar-link">列表&amp;Key</a></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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">条件渲染<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="_06-条件渲染"><a href="#_06-条件渲染" class="header-anchor">#</a> 06. 条件渲染</h1> <p>React 中的条件渲染和 JavaScript 中的一样，使用 JavaScript 运算符 <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/if...else" target="_blank" rel="noopener noreferrer"><code>if</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> 或者<a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Conditional_Operator" target="_blank" rel="noopener noreferrer">条件运算符<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>去创建元素来表现当前的状态，然后让 React 根据它们来更新 UI。</p> <p>观察这两个组件:</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">UserGreeting</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    26. 

  26.   <span class="token keyword">return</span> <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>h1</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">Welcome back!</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>h1</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">;</span>
    27. 

  27. <span class="token punctuation">}</span>
    28. 

  28. 

  29. <span class="token keyword">function</span> <span class="token function">GuestGreeting</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    30. 

  30.   <span class="token keyword">return</span> <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>h1</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">Please sign up.</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>h1</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">;</span>
    31. 

  31. <span class="token punctuation">}</span>
    32. 

  32. </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>再创建一个 <code>Greeting</code> 组件，它会根据用户是否登录来决定显示上面的哪一个组件。</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">Greeting</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    33. 

  33.   <span class="token keyword">const</span> isLoggedIn <span class="token operator">=</span> props<span class="token punctuation">.</span>isLoggedIn<span class="token punctuation">;</span>
    34. 

  34. 

  35.   <span class="token comment">// 根据条件来渲染组件(UI)</span>
    36. 

  36.   <span class="token keyword">if</span> <span class="token punctuation">(</span>isLoggedIn<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    37. 

  37.     <span class="token keyword">return</span> <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">UserGreeting</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">;</span>
    38. 

  38.   <span class="token punctuation">}</span>
    39. 

  39.   <span class="token keyword">return</span> <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">GuestGreeting</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">;</span>
    40. 

  40. <span class="token punctuation">}</span>
    41. 

  41. 

  42. ReactDOM<span class="token punctuation">.</span><span class="token function">render</span><span class="token punctuation">(</span>
    43. 

  43.   <span class="token comment">// Try changing to isLoggedIn={true}:</span>
    44. 

  44.   <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">Greeting</span></span> <span class="token attr-name">isLoggedIn</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span><span class="token boolean">false</span><span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">,</span>
    45. 

  45.   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>
    46. 

  46. <span class="token punctuation">)</span><span class="token punctuation">;</span>
    47. 

  47. </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></div></div><h2 id="元素变量"><a href="#元素变量" class="header-anchor">#</a> 元素变量</h2> <p>你可以使用变量来储存元素。 它可以帮助你有条件地渲染组件的一部分，而其他的渲染部分并不会因此而改变。</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">class</span> <span class="token class-name">LoginControl</span> <span class="token keyword">extends</span> <span class="token class-name">React<span class="token punctuation">.</span>Component</span> <span class="token punctuation">{</span>
    48. 

  48.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    49. 

  49.     <span class="token keyword">super</span><span class="token punctuation">(</span>props<span class="token punctuation">)</span><span class="token punctuation">;</span>
    50. 

  50.     <span class="token keyword">this</span><span class="token punctuation">.</span>handleLoginClick <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">handleLoginClick</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    51. 

  51.     <span class="token keyword">this</span><span class="token punctuation">.</span>handleLogoutClick <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">handleLogoutClick</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    52. 

  52.     <span class="token keyword">this</span><span class="token punctuation">.</span>state <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">isLoggedIn</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    53. 

  53.   <span class="token punctuation">}</span>
    54. 

  54. 

  55.   <span class="token function">handleLoginClick</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    56. 

  56.     <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">setState</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">isLoggedIn</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>
    57. 

  57.   <span class="token punctuation">}</span>
    58. 

  58. 

  59.   <span class="token function">handleLogoutClick</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    60. 

  60.     <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">setState</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">isLoggedIn</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    61. 

  61.   <span class="token punctuation">}</span>
    62. 

  62. 

  63.   <span class="token function">render</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    64. 

  64.     <span class="token keyword">const</span> isLoggedIn <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>state<span class="token punctuation">.</span>isLoggedIn<span class="token punctuation">;</span>
    65. 

  65.     <span class="token keyword">let</span> button<span class="token punctuation">;</span>
    66. 

  66.     <span class="token keyword">if</span> <span class="token punctuation">(</span>isLoggedIn<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    67. 

  67.       button <span class="token operator">=</span> <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">LogoutButton</span></span> <span class="token attr-name">onClick</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span><span class="token keyword">this</span><span class="token punctuation">.</span>handleLogoutClick<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">;</span>
    68. 

  68.     <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    69. 

  69.       button <span class="token operator">=</span> <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">LoginButton</span></span> <span class="token attr-name">onClick</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span><span class="token keyword">this</span><span class="token punctuation">.</span>handleLoginClick<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">;</span>
    70. 

  70.     <span class="token punctuation">}</span>
    71. 

  71. 

  72.     <span class="token keyword">return</span> <span class="token punctuation">(</span>
    73. 

  73.       <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">
    74. 

  74.         </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">Greeting</span></span> <span class="token attr-name">isLoggedIn</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>isLoggedIn<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span><span class="token plain-text">
    75. 

  75.         </span><span class="token punctuation">{</span>button<span class="token punctuation">}</span><span class="token plain-text">
    76. 

  76.       </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
    77. 

  77.     <span class="token punctuation">)</span><span class="token punctuation">;</span>
    78. 

  78.   <span class="token punctuation">}</span>
    79. 

  79. <span class="token punctuation">}</span>
    80. 

  80. 

  81. ReactDOM<span class="token punctuation">.</span><span class="token function">render</span><span class="token punctuation">(</span>
    82. 

  82.   <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">LoginControl</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">,</span>
    83. 

  83.   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>
    84. 

  84. <span class="token punctuation">)</span><span class="token punctuation">;</span>
    85. 

  85. </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><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br></div></div><p><a href="https://codepen.io/gaearon/pen/QKzAgB?editors=0010" target="_blank" rel="noopener noreferrer"><strong>在 CodePen 上尝试</strong><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> <blockquote><p>此方式有些复杂~</p></blockquote> <h2 id="与运算符"><a href="#与运算符" class="header-anchor">#</a> 与运算符 &amp;&amp;</h2> <p>通过花括号包裹代码，你可以<a href="https://zh-hans.reactjs.org/docs/introducing-jsx.html#embedding-expressions-in-jsx" target="_blank" rel="noopener noreferrer">在 JSX 中嵌入表达式<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>。这也包括 JavaScript 中的逻辑与 (&amp;&amp;) 运算符。它可以很方便地进行元素的条件渲染：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">Mailbox</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    86. 

  86.   <span class="token keyword">const</span> unreadMeg <span class="token operator">=</span> props<span class="token punctuation">.</span>unreadMeg
    87. 

  87.   <span class="token keyword">return</span> <span class="token punctuation">(</span>
    88. 

  88.   	<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">
    89. 

  89.     	</span><span class="token punctuation">{</span>
    90. 

  90.         unreadMeg<span class="token punctuation">.</span>length <span class="token operator">&amp;&amp;</span>
    91. 

  91.           <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>h2</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">You have </span><span class="token punctuation">{</span>unreadMeg<span class="token punctuation">.</span>length<span class="token punctuation">}</span><span class="token plain-text"> unread messages</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>h2</span><span class="token punctuation">&gt;</span></span>
    92. 

  92.       <span class="token punctuation">}</span><span class="token plain-text">
    93. 

  93.     </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
    94. 

  94.   <span class="token punctuation">)</span>
    95. 

  95. <span class="token punctuation">}</span>
    96. 

  96. 

  97. <span class="token keyword">const</span> meg <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'1'</span><span class="token punctuation">,</span><span class="token string">'22'</span><span class="token punctuation">,</span><span class="token string">'33'</span><span class="token punctuation">]</span>
    98. 

  98. ReactDOM<span class="token punctuation">.</span><span class="token function">render</span><span class="token punctuation">(</span>
    99. 

  99. 	<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">Mailbox</span></span> <span class="token attr-name">unreadMeg</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span>meg<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">,</span>
    100. 

  100.   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>
    101. 

  101. <span class="token punctuation">)</span>
    102. 

  102. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>如果条件是 <code>true</code>，<code>&amp;&amp;</code> 右侧的元素就会被渲染，如果是 <code>false</code>，React 会忽略并跳过它。</p> <p>请<strong>注意</strong>，返回 false 的表达式会使 <code>&amp;&amp;</code> 后面的元素被跳过，但会返回 false 表达式。在下面示例中，render 方法的返回值是 <code>&lt;div&gt;0&lt;/div&gt;</code>。</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token function">render</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    103. 

  103.   <span class="token keyword">const</span> count <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    104. 

  104.   <span class="token keyword">return</span> <span class="token punctuation">(</span>
    105. 

  105.     <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">
    106. 

  106.       // 这里会渲染count的值
    107. 

  107.       </span><span class="token punctuation">{</span> count <span class="token operator">&amp;&amp;</span> <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>h1</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">Messages: </span><span class="token punctuation">{</span>count<span class="token punctuation">}</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>h1</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">}</span><span class="token plain-text">
    108. 

  108.     </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
    109. 

  109.   <span class="token punctuation">)</span><span class="token punctuation">;</span>
    110. 

  110. <span class="token punctuation">}</span>
    111. 

  111. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><h2 id="三目运算符"><a href="#三目运算符" class="header-anchor">#</a> 三目运算符</h2> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token function">render</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    112. 

  112.   <span class="token keyword">const</span> isLoggedIn <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>state<span class="token punctuation">.</span>isLoggedIn<span class="token punctuation">;</span>
    113. 

  113.   <span class="token keyword">return</span> <span class="token punctuation">(</span>
    114. 

  114.     <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">
    115. 

  115.       The user is </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>b</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>isLoggedIn <span class="token operator">?</span> <span class="token string">'currently'</span> <span class="token operator">:</span> <span class="token string">'not'</span><span class="token punctuation">}</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>b</span><span class="token punctuation">&gt;</span></span><span class="token plain-text"> logged in.
    116. 

  116.     </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
    117. 

  117.   <span class="token punctuation">)</span><span class="token punctuation">;</span>
    118. 

  118. <span class="token punctuation">}</span>
    119. 

  119. </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><div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token function">render</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    120. 

  120.   <span class="token keyword">const</span> isLoggedIn <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>state<span class="token punctuation">.</span>isLoggedIn<span class="token punctuation">;</span>
    121. 

  121.   <span class="token keyword">return</span> <span class="token punctuation">(</span>
    122. 

  122.     <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">
    123. 

  123.       </span><span class="token punctuation">{</span>isLoggedIn
    124. 

  124.         <span class="token operator">?</span> <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">LogoutButton</span></span> <span class="token attr-name">onClick</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span><span class="token keyword">this</span><span class="token punctuation">.</span>handleLogoutClick<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span>
    125. 

  125.         <span class="token operator">:</span> <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">LoginButton</span></span> <span class="token attr-name">onClick</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span><span class="token keyword">this</span><span class="token punctuation">.</span>handleLoginClick<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span>
    126. 

  126.       <span class="token punctuation">}</span><span class="token plain-text">
    127. 

  127.     </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
    128. 

  128.   <span class="token punctuation">)</span><span class="token punctuation">;</span>
    129. 

  129. <span class="token punctuation">}</span>
    130. 

  130. </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><blockquote><p>可通过三目运算符来条件渲染相应组件。</p></blockquote> <h2 id="阻止组件渲染"><a href="#阻止组件渲染" class="header-anchor">#</a> 阻止组件渲染</h2> <p>通过让 <code>render</code> 方法直接返回 <code>null</code>阻止渲染：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">WarningBanner</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    131. 

  131.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>props<span class="token punctuation">.</span>warn<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    132. 

  132.     <span class="token comment">// 返回null则不渲染此组件</span>
    133. 

  133.     <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    134. 

  134.   <span class="token punctuation">}</span>
    135. 

  135.   <span class="token keyword">return</span> <span class="token punctuation">(</span>
    136. 

  136.     <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>div</span> <span class="token attr-name">className</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>warning<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
    137. 

  137.       Warning!
    138. 

  138.     </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
    139. 

  139.   <span class="token punctuation">)</span><span class="token punctuation">;</span>
    140. 

  140. <span class="token punctuation">}</span>
    141. 

  141. 

  142. <span class="token keyword">class</span> <span class="token class-name">Page</span> <span class="token keyword">extends</span> <span class="token class-name">React<span class="token punctuation">.</span>Component</span> <span class="token punctuation">{</span>
    143. 

  143.   <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    144. 

  144.     <span class="token keyword">super</span><span class="token punctuation">(</span>props<span class="token punctuation">)</span><span class="token punctuation">;</span>
    145. 

  145.     <span class="token keyword">this</span><span class="token punctuation">.</span>state <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">showWarning</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    146. 

  146.     <span class="token keyword">this</span><span class="token punctuation">.</span>handleToggleClick <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">handleToggleClick</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    147. 

  147.   <span class="token punctuation">}</span>
    148. 

  148. 

  149.   <span class="token function">handleToggleClick</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    150. 

  150.     <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">setState</span><span class="token punctuation">(</span><span class="token parameter">state</span> <span class="token operator">=&gt;</span> <span class="token punctuation">(</span><span class="token punctuation">{</span>
    151. 

  151.       <span class="token literal-property property">showWarning</span><span class="token operator">:</span> <span class="token operator">!</span>state<span class="token punctuation">.</span>showWarning
    152. 

  152.     <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    153. 

  153.   <span class="token punctuation">}</span>
    154. 

  154. 

  155.   <span class="token function">render</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    156. 

  156.     <span class="token keyword">return</span> <span class="token punctuation">(</span>
    157. 

  157.       <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">
    158. 

  158.         </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">WarningBanner</span></span> <span class="token attr-name">warn</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span><span class="token keyword">this</span><span class="token punctuation">.</span>state<span class="token punctuation">.</span>showWarning<span class="token punctuation">}</span></span> <span class="token punctuation">/&gt;</span></span><span class="token plain-text">
    159. 

  159.         </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>button</span> <span class="token attr-name">onClick</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span><span class="token keyword">this</span><span class="token punctuation">.</span>handleToggleClick<span class="token punctuation">}</span></span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
    160. 

  160.           </span><span class="token punctuation">{</span><span class="token keyword">this</span><span class="token punctuation">.</span>state<span class="token punctuation">.</span>showWarning <span class="token operator">?</span> <span class="token string">'Hide'</span> <span class="token operator">:</span> <span class="token string">'Show'</span><span class="token punctuation">}</span><span class="token plain-text">
    161. 

  161.         </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>button</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
    162. 

  162.       </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
    163. 

  163.     <span class="token punctuation">)</span><span class="token punctuation">;</span>
    164. 

  164.   <span class="token punctuation">}</span>
    165. 

  165. <span class="token punctuation">}</span>
    166. 

  166. 

  167. ReactDOM<span class="token punctuation">.</span><span class="token function">render</span><span class="token punctuation">(</span>
    168. 

  168.   <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span><span class="token class-name">Page</span></span> <span class="token punctuation">/&gt;</span></span><span class="token punctuation">,</span>
    169. 

  169.   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>
    170. 

  170. <span class="token punctuation">)</span><span class="token punctuation">;</span>
    171. 

  171. </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><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br><span class="line-number">39</span><br><span class="line-number">40</span><br><span class="line-number">41</span><br></div></div><p>在组件的 <code>render</code> 方法中返回 <code>null</code> 并不会影响组件的生命周期。例如，上面这个示例中，<code>componentDidUpdate</code> 依然会被调用。</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/06.条件渲染.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/f27775/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">事件处理</div></a> <a href="/blog/pages/88f4b0/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">列表&amp;Key</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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    175. 

  175.             Git修改分支名
    176. 

  176.             <!----></div></a> <span class="date">08-11</span></dt></dl><dl><dd>02</dd> <dt><a href="/blog/pages/55f894/"><div>
    177. 

  177.             CSS给table的tbody添加滚动条
    178. 

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    179. 

  179.             我做了一个手写春联小网页，祝大家虎年暴富
    180. 

  180.             <span class="title-tag">
    181. 

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    182. 

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