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<p>Async/Await是一种允许我们像构建没有回调函数的普通函数一样构建Promise的新语法。以往的异步方法无外乎回调函数和<code>Promise</code>。但是Async/Await建立于<a href="https://mydearest.cn/createPromise.html">Promise</a>之上。</p>
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<h2 id="Async-Await语法"><a href="#Async-Await语法" class="headerlink" title="Async/Await语法"></a>Async/Await语法</h2><h3 id="async关键字-放置在一个函数前面-。"><a href="#async关键字-放置在一个函数前面-。" class="headerlink" title="async关键字(放置在一个函数前面)。"></a>async关键字(放置在一个函数前面)。</h3><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">async</span> <span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="number">1</span></span><br><span class="line"> <span class="comment">// return Promise.resolve(1)</span></span><br><span class="line">}</span><br><span class="line">f().then(value) <span class="comment">// 1</span></span><br></pre></td></tr></table></figure>
<p>async修饰过的函数总是返回一个promise，如果代码中有return <非promise>语句，JavaScript会自动把返回的这个value值包装成promise的resolved值。</p>
<h3 id="await关键字-只能在async函数内部使用"><a href="#await关键字-只能在async函数内部使用" class="headerlink" title="await关键字(只能在async函数内部使用)"></a>await关键字(只能在async函数内部使用)</h3><p>关键词await可以让JavaScript进行等待，直到一个promise执行并返回它的结果，JavaScript才会继续往下执行。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">timeout</span>(<span class="params">ms</span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve</span>) =></span> {</span><br><span class="line"> setTimeout(resolve, ms);</span><br><span class="line"> });</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">async</span> <span class="function"><span class="keyword">function</span> <span class="title">asyncPrint</span>(<span class="params">value, ms</span>) </span>{</span><br><span class="line"> <span class="keyword">await</span> timeout(ms);</span><br><span class="line"> <span class="built_in">console</span>.log(value);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">asyncPrint(<span class="string">'hello world'</span>, <span class="number">50</span>);</span><br></pre></td></tr></table></figure>
<h3 id="错误处理-使用try-catch捕获-可以给每个-await-后的-Promise-增加-catch-方法；也可以将-await-的代码放在-try…catch-中"><a href="#错误处理-使用try-catch捕获-可以给每个-await-后的-Promise-增加-catch-方法；也可以将-await-的代码放在-try…catch-中" class="headerlink" title="错误处理(使用try-catch捕获)可以给每个 await 后的 Promise 增加 catch 方法；也可以将 await 的代码放在 try…catch 中"></a>错误处理(使用try-catch捕获)可以给每个 await 后的 Promise 增加 catch 方法；也可以将 await 的代码放在 try…catch 中</h3><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">async</span> <span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">let</span> response = <span class="keyword">await</span> fetch(<span class="string">'http://no-such-url'</span>)</span><br><span class="line"> } <span class="keyword">catch</span> (err) {</span><br><span class="line"> alet(err) <span class="comment">// TypeError: failed to fetch 也能捕获多行语句 类似链式Promise最后的单个catch函数</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line">f()</span><br></pre></td></tr></table></figure>
<p>如果我们不使用try-catch，然后async函数f()的调用产生的promise变成reject状态的话，我们可以添加.catch去处理它：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">async</span> <span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">let</span> response = <span class="keyword">await</span> fetch(<span class="string">'http://no-such-url'</span>)</span><br><span class="line">}</span><br><span class="line"><span class="comment">// f()变成了一个rejected的promise</span></span><br><span class="line">f().catch(alert) <span class="comment">// TypeError: failed to fetch</span></span><br></pre></td></tr></table></figure>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">async</span> <span class="function"><span class="keyword">function</span> <span class="title">getData</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">if</span> (<span class="literal">true</span>) {</span><br><span class="line"> <span class="keyword">throw</span> <span class="built_in">Error</span>(<span class="string">"Catch me if you can"</span>);</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">catch</span> (err) {</span><br><span class="line"> <span class="built_in">console</span>.log(err.message);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line">getData()</span><br><span class="line"> .then(<span class="function"><span class="params">()</span> =></span> <span class="built_in">console</span>.log(<span class="string">"I will run no matter what!"</span>))</span><br><span class="line"> .catch(<span class="function"><span class="params">()</span> =></span> <span class="built_in">console</span>.log(<span class="string">"Catching err"</span>));</span><br></pre></td></tr></table></figure>
<p>throw 抛出的错误永远不会触发<strong>getData()</strong>的catch方法。</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">async</span> <span class="function"><span class="keyword">function</span> <span class="title">getData</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">if</span> (<span class="literal">true</span>) {</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">Promise</span>.reject(<span class="string">"Catch me if you can"</span>);</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">catch</span> (err) {</span><br><span class="line"> <span class="built_in">console</span>.log(err.message);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="结合Promise-all"><a href="#结合Promise-all" class="headerlink" title="结合Promise.all"></a>结合Promise.all</h3><p>async/await能够与Promise.all友好的协作，当我们需要等待多个promise时，我们可以将他们包装在Promise.all中然后使用await：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 直到数组全部返回结果</span></span><br><span class="line"><span class="keyword">let</span> results = <span class="keyword">await</span> <span class="built_in">Promise</span>.all([</span><br><span class="line"> fetch(url1),</span><br><span class="line"> fetch(url2),</span><br><span class="line"> ...</span><br><span class="line">])</span><br></pre></td></tr></table></figure>
<h2 id="简单实现async函数"><a href="#简单实现async函数" class="headerlink" title="简单实现async函数"></a>简单实现async函数</h2><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">spawn</span>(<span class="params">genF</span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>{</span><br><span class="line"> <span class="keyword">const</span> gen = genF();</span><br><span class="line"> <span class="function"><span class="keyword">function</span> <span class="title">step</span>(<span class="params">nextF</span>) </span>{</span><br><span class="line"> <span class="keyword">let</span> next;</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> next = nextF();</span><br><span class="line"> } <span class="keyword">catch</span> (e) {</span><br><span class="line"> <span class="keyword">return</span> reject(e);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span> (next.done) {</span><br><span class="line"> <span class="keyword">return</span> resolve(next.value);</span><br><span class="line"> }</span><br><span class="line"> <span class="built_in">Promise</span>.resolve(next.value).then(</span><br><span class="line"> <span class="function"><span class="keyword">function</span>(<span class="params">v</span>) </span>{</span><br><span class="line"> step(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> gen.next(v);</span><br><span class="line"> });</span><br><span class="line"> },</span><br><span class="line"> <span class="function"><span class="keyword">function</span>(<span class="params">e</span>) </span>{</span><br><span class="line"> step(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> gen.throw(e);</span><br><span class="line"> });</span><br><span class="line"> }</span><br><span class="line"> );</span><br><span class="line"> }</span><br><span class="line"> step(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> gen.next(<span class="literal">undefined</span>);</span><br><span class="line"> });</span><br><span class="line"> });</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2 id="Promises-组合：Promise-all，Promise-allSettled，-Promise-any"><a href="#Promises-组合：Promise-all，Promise-allSettled，-Promise-any" class="headerlink" title="Promises 组合：Promise.all，Promise.allSettled， Promise.any"></a>Promises 组合：Promise.all，Promise.allSettled， Promise.any</h2><ul>
<li><p>Promise API 提供了许多将Promise组合在一起的方法。 其中最有用的是Promise.all，它接受一个Promises数组并返回一个Promise。 如果参数中 promise 有一个失败（rejected），此实例回调失败（reject），失败原因的是第一个失败 promise 的结果。</p>
</li>
<li><p>Promise.race(iterable) 方法返回一个 promise，一旦迭代器中的某个promise解决或拒绝，返回的 promise就会解决或拒绝。</p>
</li>
<li><p>较新版本的V8也将实现两个新的组合：Promise.allSettled和Promise.any。 Promise.any仍然处于提案的早期阶段：在撰写本文时，仍然没有浏览器支持它。</p>
</li>
</ul>
<p>Promise.any可以表明任何Promise是否fulfilled。 与 Promise.race的区别在于Promise.any不会拒绝即使其中一个Promise被拒绝。</p>
<p>无论如何，两者中最有趣的是 Promise.allSettled，它也是 Promise 数组，但如果其中一个Promise拒绝，它不会短路。 当你想要检查Promise数组是否全部已解决时，它是有用的，无论最终是否拒绝，可以把它想象成Promise.all 的反对者。</p>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>随着单页JavaScript web程序的兴起和对NodeJS的广泛采用，如何优雅的处理并发对于JavaScript开发人员来说比任何以往的时候都显得更为重要。</p>
<p>JS引擎是单线程的，这意味着运行函数只有一个调用堆栈。这一限制是JS异步本质的基础:所有需要时间的操作都必须由外部实体(例如浏览器)或回调函数负责。</p>
<p><code>Async/Await</code>缓解了许多因为控制流问题而导致bug遍地的这个困扰着JavaScript代码库数十年的问题，并且几乎可以保证让任何异步代码块变的更精炼，更简单，更自信。</p>
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<p><span>本文标题:</span><a href="/es7async.html">使用 Async/Await 让你的代码更简洁</a></p>
<p><span>文章作者:</span><a href="/" title="访问 陈宇（cosyer） 的个人博客">陈宇（cosyer）</a></p>
<p><span>发布时间:</span>2018年06月11日 - 01:06</p>
<p><span>最后更新:</span>2020年08月24日 - 21:08</p>
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<div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-2"><a class="nav-link" href="#Async-Await语法"><span class="nav-number">1.</span> <span class="nav-text">Async/Await语法</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#async关键字-放置在一个函数前面-。"><span class="nav-number">1.1.</span> <span class="nav-text">async关键字(放置在一个函数前面)。</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#await关键字-只能在async函数内部使用"><span class="nav-number">1.2.</span> <span class="nav-text">await关键字(只能在async函数内部使用)</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#错误处理-使用try-catch捕获-可以给每个-await-后的-Promise-增加-catch-方法；也可以将-await-的代码放在-try…catch-中"><span class="nav-number">1.3.</span> <span class="nav-text">错误处理(使用try-catch捕获)可以给每个 await 后的 Promise 增加 catch 方法；也可以将 await 的代码放在 try…catch 中</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#结合Promise-all"><span class="nav-number">1.4.</span> <span class="nav-text">结合Promise.all</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#简单实现async函数"><span class="nav-number">2.</span> <span class="nav-text">简单实现async函数</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#Promises-组合：Promise-all，Promise-allSettled，-Promise-any"><span class="nav-number">3.</span> <span class="nav-text">Promises 组合：Promise.all，Promise.allSettled， Promise.any</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#总结"><span class="nav-number">4.</span> <span class="nav-text">总结</span></a></li></ol></div>
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