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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"><span>初识 TypeScript</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading open"><span>TypeScript 常用语法</span> <span class="arrow down"></span></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/blog/pages/28672e2743bbc3a7/" class="sidebar-link">基础类型</a></li><li><a href="/blog/pages/54add7f5cf78088e/" class="sidebar-link">变量声明</a></li><li><a href="/blog/pages/54ea89b497ec3bb3/" class="sidebar-link">接口</a></li><li><a href="/blog/pages/fad060bd9a8bfac6/" class="sidebar-link">类</a></li><li><a href="/blog/pages/870a51ba2a9edfad/" class="sidebar-link">函数</a></li><li><a href="/blog/pages/8045759ec4ad3c01/" class="sidebar-link">泛型</a></li><li><a href="/blog/pages/7279420c899c505d/" aria-current="page" class="active sidebar-link">类型推断</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/blog/pages/7279420c899c505d/#基础" class="sidebar-link">基础</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7279420c899c505d/#最佳通用类型" class="sidebar-link">最佳通用类型</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/7279420c899c505d/#上下文类型" class="sidebar-link">上下文类型</a></li></ul></li><li><a href="/blog/pages/3fb6c2f52ab398e3/" class="sidebar-link">高级类型</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ts-axios 项目初始化</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ts-axios 基础功能实现</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ts-axios 异常情况处理</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ts-axios 接口扩展</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ts-axios 拦截器实现</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ts-axios 配置化实现</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ts-axios 取消功能实现</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ts-axios 更多功能实现</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ts-axios 单元测试</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>ts-axios 部署与发布</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/typescript-axios/#《TypeScript 从零实现 axios》" data-v-06970110>《TypeScript 从零实现 axios》</a></li><li data-v-06970110><a href="/blog/note/typescript-axios/#TypeScript 常用语法" data-v-06970110>TypeScript 常用语法</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="javascript:;" data-v-06970110>HuangYi</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2020-01-05</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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">类型推断<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="类型推断"><a href="#类型推断" class="header-anchor">#</a> 类型推断</h1> <p>这节介绍 TypeScript 里的类型推断。即，类型是在哪里如何被推断的。</p> <h2 id="基础"><a href="#基础" class="header-anchor">#</a> 基础</h2> <p>TypeScript 里，在有些没有明确指出类型的地方，类型推断会帮助提供类型。如下面的例子：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token number">3</span>
    26. 

  26. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>变量 <code>x</code> 的类型被推断为数字。 这种推断发生在初始化变量和成员，设置默认参数值和决定函数返回值时。</p> <p>大多数情况下，类型推断是直截了当地。后面的小节，我们会浏览类型推断时的细微差别。</p> <h2 id="最佳通用类型"><a href="#最佳通用类型" class="header-anchor">#</a> 最佳通用类型</h2> <p>有些时候我们需要从几个表达式中推断类型，会使用这些表达式的类型来推断出一个最合适的通用类型。例如，</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">]</span>
    27. 

  27. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>为了推断 <code>x</code> 的类型，我们必须考虑所有元素的类型。 这里有两种选择：<code>number</code> 和 <code>null</code>。 计算通用类型算法会考虑所有的候选类型，并给出一个兼容所有候选类型的类型。</p> <p>由于最终的通用类型取自候选类型，有些时候候选类型共享一个公共结构，但是却没有一个类型能做为所有候选类型的超级类型。例如：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">class</span> <span class="token class-name">Animal</span> <span class="token punctuation">{</span>
    28. 

  28.   numLegs<span class="token operator">:</span> <span class="token builtin">number</span>
    29. 

  29. <span class="token punctuation">}</span>
    30. 

  30. 

  31. <span class="token keyword">class</span> <span class="token class-name">Bee</span> <span class="token keyword">extends</span> <span class="token class-name">Animal</span> <span class="token punctuation">{</span>
    32. 

  32. <span class="token punctuation">}</span>
    33. 

  33. 

  34. <span class="token keyword">class</span> <span class="token class-name">Lion</span> <span class="token keyword">extends</span> <span class="token class-name">Animal</span> <span class="token punctuation">{</span>
    35. 

  35. <span class="token punctuation">}</span>
    36. 

  36. 

  37. <span class="token keyword">let</span> zoo <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token keyword">new</span> <span class="token class-name">Bee</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token keyword">new</span> <span class="token class-name">Lion</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span>
    38. 

  38. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>这里，我们想让 <code>zoo</code> 被推断为 <code>Animal[]</code> 类型，但是这个数组里没有对象是 <code>Animal</code> 类型的，因此不能推断出这个结果。 为了更正，我们可以明确的声明我们期望的类型：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">let</span> zoo<span class="token operator">:</span> Animal<span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token keyword">new</span> <span class="token class-name">Bee</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token keyword">new</span> <span class="token class-name">Lion</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span>
    39. 

  39. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果没有找到最佳通用类型的话，类型推断的结果为联合数组类型，<code>(Bee | Lion)[]</code></p> <h2 id="上下文类型"><a href="#上下文类型" class="header-anchor">#</a> 上下文类型</h2> <p>有些时候，TypeScript 类型推断会按另外一种方式，我们称作“上下文类型”；上下文类型的出现和表达式的类型以及所处的位置相关。比如：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code>window<span class="token punctuation">.</span><span class="token function-variable function">onmousedown</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span>mouseEvent<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    40. 

  40.   <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>mouseEvent<span class="token punctuation">.</span>clickTime<span class="token punctuation">)</span>  <span class="token comment">// Error</span>
    41. 

  41. <span class="token punctuation">}</span>
    42. 

  42. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>这个例子会得到一个类型错误，TypeScript 类型检查器使用 <code>window.onmousedown</code> 函数的类型来推断右边函数表达式的类型。 因此，就能推断出 <code>mouseEvent</code> 参数的类型了，所以 <code>mouseEvent</code> 访问了一个不存在的属性，就报错了。</p> <p>如果上下文类型表达式包含了明确的类型信息，上下文的类型被忽略。重写上面的例子：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code>window<span class="token punctuation">.</span><span class="token function-variable function">onmousedown</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span>mouseEvent<span class="token operator">:</span><span class="token builtin">any</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    43. 

  43.   <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>mouseEvent<span class="token punctuation">.</span>clickTime<span class="token punctuation">)</span>  <span class="token comment">// OK</span>
    44. 

  44. <span class="token punctuation">}</span>
    45. 

  45. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>这个函数表达式有明确的参数类型注解，上下文类型被忽略。这样的话就不报错了，因为这里不会使用到上下文类型。</p> <p>上下文类型会在很多情况下使用到。通常包含函数的参数，赋值表达式的右边，类型断言，对象成员，数组字面量和返回值语句。上下文类型也会做为最佳通用类型的候选类型。比如：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">function</span> <span class="token function">createZoo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">:</span> Animal<span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token punctuation">{</span>
    46. 

  46.   <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token keyword">new</span> <span class="token class-name">Bee</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token keyword">new</span> <span class="token class-name">Lion</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span>
    47. 

  47. <span class="token punctuation">}</span>
    48. 

  48. 

  49. <span class="token keyword">let</span> zoo <span class="token operator">=</span> <span class="token function">createZoo</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    50. 

  50. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>这个例子里，最佳通用类型有 <code>3</code> 个候选者：<code>Animal</code>，<code>Bee</code> 和 <code>Lion</code>。 其中，<code>Animal</code> 会被做为最佳通用类型。</p></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《TypeScript 从零实现 axios》/02.TypeScript 常用语法/07.类型推断.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=TypeScript" title="标签">#TypeScript</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/8045759ec4ad3c01/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">泛型</div></a> <a href="/blog/pages/3fb6c2f52ab398e3/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">高级类型</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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