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[js]当async与await遇上循环.html
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<svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" 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></a></div> <!----></nav> <ul class="sidebar-links"><li><section class="sidebar-group depth-0"><p class="sidebar-heading open"><span>当async与await遇上循环</span> <!----></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/idea-domain/post/%5Bjs%5D%E5%BD%93async%E4%B8%8Eawait%E9%81%87%E4%B8%8A%E5%BE%AA%E7%8E%AF.html#循环中的-await" class="sidebar-link">循环中的 await</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/idea-domain/post/%5Bjs%5D%E5%BD%93async%E4%B8%8Eawait%E9%81%87%E4%B8%8A%E5%BE%AA%E7%8E%AF.html#总结" class="sidebar-link">总结</a><ul class="sidebar-sub-headers"></ul></li></ul></section></li></ul> </aside> <div class="password-shadow password-wrapper-in" style="display:none;" data-v-7c5ab02c data-v-5f3f4765><h3 class="title" style="display:none;" data-v-7c5ab02c data-v-7c5ab02c>当async与await遇上循环</h3> <!----> <label id="box" class="inputBox" style="display:none;" data-v-7c5ab02c data-v-7c5ab02c><input type="password" value="" data-v-7c5ab02c> <span data-v-7c5ab02c>Konck! Knock!</span> <button data-v-7c5ab02c>OK</button></label> <div class="footer" style="display:none;" data-v-7c5ab02c data-v-7c5ab02c><span data-v-7c5ab02c><i class="iconfont reco-theme" data-v-7c5ab02c></i> <a target="blank" href="https://vuepress-theme-reco.recoluan.com" data-v-7c5ab02c>vuePress-theme-reco</a></span> <span data-v-7c5ab02c><i class="iconfont reco-copyright" data-v-7c5ab02c></i> <a data-v-7c5ab02c><span data-v-7c5ab02c>JyLie</span>
<span data-v-7c5ab02c>2017 - </span>
2023
</a></span></div></div> <div data-v-5f3f4765><main class="page"><div class="page-title" style="display:none;"><h1>当async与await遇上循环</h1> <hr> <div data-v-2c863f0e><i class="iconfont reco-account" data-v-2c863f0e><span data-v-2c863f0e>JyLie</span></i> <i class="iconfont reco-date" data-v-2c863f0e><span data-v-2c863f0e>2019-08-08</span></i> <!----> <i class="iconfont reco-tag tags" data-v-2c863f0e><span class="tag-item" data-v-2c863f0e>es6</span></i></div></div> <div class="theme-reco-content content__default" style="display:none;"><h2 id="循环中的-await"><a href="#循环中的-await" class="header-anchor">#</a> 循环中的 await</h2> <p>在一次项目中遇到使用 async 函数来循环遍历数组，其中每个数组元素都是一个异步图片上传函数的情景。</p> <p>过了一会测试发现，通过 forEach 或 map 来套用 async 函数进行循环时，上传接口数据会有重复覆盖的情况，这是因为 forEach\map 本身就是就是异步函数</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">let</span> list <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 number">6</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">function</span> <span class="token function">fun</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">//这里其实是调用接口，所以时间完全无法把握</span>
<span class="token keyword">return</span> item<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">go1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
list<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">{</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>item<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> item1 <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fun</span><span class="token punctuation">(</span>item<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>item1<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//加上这句话</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'done'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token function">go1</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 done 1 2 3 4 5 6</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></div></div><blockquote><p>从运行结果中，可以看出，代码没有等待 forEach 方法执行结束，而是直接继续往后执行了。回调函数传进去之后，2 秒钟之后才会返回数据，这不是我们想要的结果，我现在是希望它能顺序的执行。</p></blockquote> <p>于是查找了下文档发现，循环异步函数时需要使用 for 循环来遍历。于是:</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">let</span> list <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 number">6</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">function</span> <span class="token function">fun</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">//这里其实是调用接口，所以时间完全无法把握</span>
<span class="token keyword">return</span> item<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">go2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> item <span class="token keyword">of</span> list<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">let</span> item1 <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fun</span><span class="token punctuation">(</span>item<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>item1<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'done'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token function">go2</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 1 2 3 4 5 6 done</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><blockquote><p>按照上述代码的打印结果分析，for 循环内的代码，每一次都会等当前的 fun 方法返回了内容之后，才会继续下一个循环，循环全部结束的时候，才会跑 for 循环下面的代码，这是我们想要的结果，nice。</p></blockquote> <h2 id="总结"><a href="#总结" class="header-anchor">#</a> 总结</h2> <ul><li>不要在 forEach 方法内使用 async、await，尽量避免这种写法，不然就。。。</li> <li>当使用 async 和 await 时，要配合 for 循环内使用</li></ul></div> <footer class="page-edit" style="display:none;"><!----> <!----></footer> <!----> <!----></main> <!----></div></div></div></div><div class="global-ui"><div class="back-to-ceiling" style="right:1rem;bottom:6rem;width:2.5rem;height:2.5rem;border-radius:.25rem;line-height:2.5rem;display:none;" data-v-a936bdb4 data-v-a936bdb4><svg t="1574745035067" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg" p-id="5404" class="icon" data-v-a936bdb4><path d="M526.60727968 10.90185116a27.675 27.675 0 0 0-29.21455937 0c-131.36607665 82.28402758-218.69155461 228.01873535-218.69155402 394.07834331a462.20625001 462.20625001 0 0 0 5.36959153 69.94390903c1.00431239 6.55289093-0.34802892 13.13561351-3.76865779 18.80351572-32.63518765 54.11355614-51.75690182 118.55860487-51.7569018 187.94566865a371.06718723 371.06718723 0 0 0 11.50484808 91.98906777c6.53300375 25.50556257 41.68394495 28.14064038 52.69160883 4.22606766 17.37162448-37.73630017 42.14135425-72.50938081 72.80769204-103.21549295 2.18761121 3.04276886 4.15646224 6.24463696 6.40373557 9.22774369a1871.4375 1871.4375 0 0 0 140.04691725 5.34970492 1866.36093723 1866.36093723 0 0 0 140.04691723-5.34970492c2.24727335-2.98310674 4.21612437-6.18497483 6.3937923-9.2178004 30.66633723 30.70611158 55.4360664 65.4791928 72.80769147 103.21549355 11.00766384 23.91457269 46.15860503 21.27949489 52.69160879-4.22606768a371.15156223 371.15156223 0 0 0 11.514792-91.99901164c0-69.36717486-19.13165746-133.82216804-51.75690182-187.92578088-3.42062944-5.66790279-4.76302748-12.26056868-3.76865837-18.80351632a462.20625001 462.20625001 0 0 0 5.36959269-69.943909c-0.00994388-166.08943902-87.32547796-311.81420293-218.6915546-394.09823051zM605.93803103 357.87693858a93.93749974 93.93749974 0 1 1-187.89594924 6.1e-7 93.93749974 93.93749974 0 0 1 187.89594924-6.1e-7z" p-id="5405" data-v-a936bdb4></path><path d="M429.50777625 765.63860547C429.50777625 803.39355007 466.44236686 1000.39046097 512.00932183 1000.39046097c45.56695499 0 82.4922232-197.00623328 82.5015456-234.7518555 0-37.75494459-36.9345906-68.35043303-82.4922232-68.34111062-45.57627738-0.00932239-82.52019037 30.59548842-82.51086798 68.34111062z" p-id="5406" data-v-a936bdb4></path></svg></div><div id="live2d-widget" class="live2d-widget-container" style="position:fixed;right:65px;bottom:0px;width:130px;height:290px;z-index:99999;opacity:0.8;pointer-events:none;"><canvas id="live2d_canvas" width="130" height="290" class="live2d_canvas" style="position:absolute;left:0px;top:0px;width:130px;height:290px;"></canvas></div></div></div>
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