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<!DOCTYPE html>
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<span itemprop="image" itemscope="" itemtype="http://schema.org/ImageObject"><img itemprop="url image" src="http://cdn.mydearest.cn/blog/images/promise.png" class="full-image" alt="Promise" title="Promise"><meta itemprop="width" content="auto"><meta itemprop="height" content="auto"></span>
<p>前端近年的兴起，有大部分是因为 <code>NodeJS</code> 的诞生，而 <code>NodeJS</code> 是个适用于 <strong>异步IO</strong> 密集型的语言，一些基于 <code>NodeJS</code> 的框架，比如 <em>KOA2、Adonis</em> 就有大量的 <code>async</code> 和 <code>await</code> 语法，<code>async</code>的函数的返回值就是 <code>Promise</code> 对象，我们可以用 <code>async</code> 和 <code>await</code> 语法，写出优雅的异步代码，来替换难看且难维护的回调函数。</p>
<h2 id="Promise-概念-JS的-Promise是未来事件的表示"><a href="#Promise-概念-JS的-Promise是未来事件的表示" class="headerlink" title="Promise 概念(JS的 Promise是未来事件的表示)"></a>Promise 概念(JS的 Promise是未来事件的表示)</h2><p><code>Promise</code>是一种对异步操作的封装，主流的规范是Promise/A+。
<code>Promise</code>可以使得异步代码层次清晰，便于理解，且更加容易维护。 Promise 可以以成功结束：用行话说我们已经解决了resolved(fulfilled)。 但如果 Promise 出错，我们会说它处于<strong>拒绝(rejected )状态。 Promise 也有一个默认状态：每个新的 Promise 都以挂起(pending)</strong>状态开始。
<code>Promise</code>构造函数接受一个函数作为参数，该函数的两个参数分别是resolve和reject。它们是两个函数，由 JavaScript 引擎提供，不用自己部署。
生成实例时回执信作为参数的函数；<Br>
<code>resolve</code>函数的作用是，将Promise对象的状态从“未完成”变为“成功”（即从 pending 变为 fulfilled），在异步操作成功时调用，并将异步操作的结果，作为参数传递出去；
<code>reject</code>函数的作用是，将Promise对象的状态从“未完成”变为“失败”（即从 pending 变为 rejected），在异步操作失败时调用，并将异步操作报出的错误，作为参数传递出去。</Br></p>
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<a id="more"></a>
<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><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 执行顺序</span></span><br><span class="line"><span class="keyword">let</span> promise = <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="built_in">console</span>.log(<span class="string">'Promise'</span>); <span class="comment">// 新建后立即执行</span></span><br><span class="line"> resolve();</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="number">111</span>)</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span></span><br><span class="line">});</span><br><span class="line">promise.then(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="string">'resolved.'</span>); <span class="comment">// 同步任务执行完成后才会执行</span></span><br><span class="line">});</span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">'Hi!'</span>);</span><br><span class="line"><span class="comment">// Promise</span></span><br><span class="line"><span class="comment">// 111</span></span><br><span class="line"><span class="comment">// Hi!</span></span><br><span class="line"><span class="comment">// resolved</span></span><br></pre></td></tr></table></figure>
<p>接下来我们就用<code>Promise</code>结合ajax来使用</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><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> getJSON = <span class="function"><span class="keyword">function</span>(<span class="params">url</span>) </span>{</span><br><span class="line"> <span class="keyword">const</span> promise = <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> handler = <span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">if</span> (<span class="keyword">this</span>.readyState !== <span class="number">4</span>) {</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span> (<span class="keyword">this</span>.status === <span class="number">200</span>) {</span><br><span class="line"> resolve(<span class="keyword">this</span>.responseText);</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> reject(<span class="keyword">new</span> <span class="built_in">Error</span>(<span class="keyword">this</span>.statusText));</span><br><span class="line"> }</span><br><span class="line"> };</span><br><span class="line"> <span class="keyword">const</span> client = <span class="keyword">new</span> XMLHttpRequest();</span><br><span class="line"> client.open(<span class="string">"GET"</span>, url);</span><br><span class="line"> client.onreadystatechange = handler;</span><br><span class="line"> client.responseType = <span class="string">"json"</span>;</span><br><span class="line"> client.setRequestHeader(<span class="string">"Accept"</span>, <span class="string">"application/json"</span>);</span><br><span class="line"> client.send();</span><br><span class="line"> });</span><br><span class="line"> <span class="keyword">return</span> promise;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line">getJSON(<span class="string">"/posts.json"</span>).then(<span class="function"><span class="keyword">function</span>(<span class="params">json</span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="string">'Contents: '</span> + json);</span><br><span class="line">}, <span class="function"><span class="keyword">function</span>(<span class="params">error</span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.error(<span class="string">'出错了'</span>, error);</span><br><span class="line">});</span><br></pre></td></tr></table></figure>
<p>这里我们会渐进式的来创建一个 <code>Promise</code> 的实现，如果，你还不了解 <code>Promise</code> ，赶快移步 <a href="http://es6.ruanyifeng.com/#docs/promise" target="_blank" rel="noopener">Promise</a> 了解学习，当然这个实现会符合 <a href="https://promisesaplus.com" target="_blank" rel="noopener">Promise/A+</a> 规范，<code>JavaScript</code> 中有很多第三方的 <code>Promise</code> 库，<a href="http://bluebirdjs.com/docs/getting-started.html" target="_blank" rel="noopener">bluebird</a> 就是一个第三方 <code>Promise</code> 类库，相比其它第三方类库或标准对象来说，其有以下优点：功能更齐全而不臃肿、浏览器兼容性更好,大家可以了解下。</p>
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<p>废话不多说，直接开干。。。 😠</p>
<h2 id="定义-Promise-类型"><a href="#定义-Promise-类型" class="headerlink" title="定义 Promise 类型"></a>定义 Promise 类型</h2><p>一个简单 <code>Promise</code> 语法，如下</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><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> promise = <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="comment">// ... doSomething</span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (<span class="comment">/* 异步操作成功 */</span>){</span><br><span class="line"> resolve(value);</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> reject(error);</span><br><span class="line"> }</span><br><span class="line">});</span><br><span class="line"></span><br><span class="line">promise.then(<span class="function"><span class="keyword">function</span>(<span class="params">value</span>) </span>{</span><br><span class="line"> <span class="comment">// success</span></span><br><span class="line">}, <span class="function"><span class="keyword">function</span>(<span class="params">error</span>) </span>{</span><br><span class="line"> <span class="comment">// failure 第二个函数可选</span></span><br><span class="line">});</span><br></pre></td></tr></table></figure>
<h3 id="实现-resolve-和-then"><a href="#实现-resolve-和-then" class="headerlink" title="实现 resolve 和 then"></a>实现 resolve 和 then</h3><p>首先我们以上 👆 的语法，自己定义一个 <code>Promise</code> 实例</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><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Promise</span>(<span class="params">fn</span>) </span>{</span><br><span class="line"> <span class="keyword">let</span> callback = <span class="literal">null</span>;</span><br><span class="line"> <span class="comment">//实现 then 方法 , 先一步一步来，实现传一个参数 -- resolve</span></span><br><span class="line"> <span class="keyword">this</span>.then = <span class="function"><span class="keyword">function</span>(<span class="params">cb</span>) </span>{</span><br><span class="line"> callback = cb;</span><br><span class="line"> };</span><br><span class="line"></span><br><span class="line"> <span class="comment">//实现 resolve , value:异步操作的最终值</span></span><br><span class="line"> <span class="function"><span class="keyword">function</span> <span class="title">resolve</span>(<span class="params">value</span>) </span>{</span><br><span class="line"> callback(value);</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//执行 function 参数</span></span><br><span class="line"> fn(resolve);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>一个简单的实例写好了，然后，来用一下，看看 👀 结果如何</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> p = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve</span>)</span>{</span><br><span class="line"> resolve(<span class="number">66</span>);</span><br><span class="line">});</span><br><span class="line"></span><br><span class="line">p.then(<span class="function"><span class="keyword">function</span>(<span class="params">value</span>)</span>{</span><br><span class="line"> <span class="built_in">console</span>.log(value);</span><br><span class="line">});</span><br></pre></td></tr></table></figure>
<div class="note info"><p>执行结果是：<code>callback is not a function</code></p></div>
<h3 id="改进1：延时resolve，修改-callback-为异步"><a href="#改进1：延时resolve，修改-callback-为异步" class="headerlink" title="改进1：延时resolve，修改 callback 为异步"></a>改进1：延时resolve，修改 callback 为异步</h3><p><strong>这里就遇到一个问题： </strong> </p>
<p><strong> 目前的Promise有一个bug，假如fn中所包含的是同步代码，则resolve会立即执行，<code>callback</code> 还是 <code>null</code> ，我们的代码是同步的，而不是异步的。<br> 如是，想办法解决掉这个问题，就是利用 <code>setTimeout</code> , 把 <code>callback</code> 加入异步队列</strong> </p>
<p>代码如下 👇</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><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Promise</span>(<span class="params">fn</span>) </span>{</span><br><span class="line"> <span class="keyword">let</span> callback = <span class="literal">null</span>;</span><br><span class="line"> <span class="comment">//实现 then 方法 , 先一步一步来，实现传一个参数 -- resolve</span></span><br><span class="line"> <span class="keyword">this</span>.then = <span class="function"><span class="keyword">function</span>(<span class="params">cb</span>) </span>{</span><br><span class="line"> callback = cb;</span><br><span class="line"> };</span><br><span class="line"></span><br><span class="line"> <span class="comment">//实现 resolve , value:异步操作的最终值</span></span><br><span class="line"> <span class="function"><span class="keyword">function</span> <span class="title">resolve</span>(<span class="params">value</span>) </span>{</span><br><span class="line"> <span class="comment">// 用 setTimeout 把 callback 加入到异步队列，这样就会，先执行 then() 方法</span></span><br><span class="line"> setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> callback(value);</span><br><span class="line"> },<span class="number">0</span>)</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//执行 function 参数</span></span><br><span class="line"> fn(resolve);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="改进2：注册多个回调函数，并实现then的链式调用"><a href="#改进2：注册多个回调函数，并实现then的链式调用" class="headerlink" title="改进2：注册多个回调函数，并实现then的链式调用"></a>改进2：注册多个回调函数，并实现then的链式调用</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><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Promise</span>(<span class="params">fn</span>) </span>{</span><br><span class="line"> <span class="keyword">let</span> value = <span class="literal">null</span></span><br><span class="line"> <span class="keyword">let</span> callbackList = [];</span><br><span class="line"> <span class="keyword">this</span>.then = <span class="function"><span class="keyword">function</span>(<span class="params">cb</span>) </span>{</span><br><span class="line"> callbackList.push(cb);</span><br><span class="line"> <span class="comment">// 实现链式调用</span></span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">this</span></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="title">resolve</span>(<span class="params">newValue</span>) </span>{</span><br><span class="line"> value = newValue</span><br><span class="line"> setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="comment">// 遍历callbackList数组依次执行</span></span><br><span class="line"> callbackList.forEach(<span class="function">(<span class="params">callback</span>)=></span>{</span><br><span class="line"> callback(value)</span><br><span class="line"> })</span><br><span class="line"> },<span class="number">0</span>)</span><br><span class="line"> }</span><br><span class="line"> fn(resolve);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="改进3：引入状态"><a href="#改进3：引入状态" class="headerlink" title="改进3：引入状态"></a>改进3：引入状态</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><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Promise</span>(<span class="params">fn</span>) </span>{</span><br><span class="line"> <span class="keyword">let</span> state=<span class="string">'pending'</span></span><br><span class="line"> <span class="keyword">let</span> value = <span class="literal">null</span></span><br><span class="line"> <span class="keyword">let</span> callbackList = [];</span><br><span class="line"> <span class="keyword">this</span>.then = <span class="function"><span class="keyword">function</span>(<span class="params">cb</span>) </span>{</span><br><span class="line"> <span class="keyword">if</span>(state==<span class="string">'pending'</span>){</span><br><span class="line"> <span class="comment">// pending加入队列</span></span><br><span class="line"> callbackList.push(cb);</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">this</span></span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span>(state==<span class="string">'fulfilled'</span>){</span><br><span class="line"> <span class="comment">// fulfilled立即执行</span></span><br><span class="line"> cb(value)</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">this</span></span><br><span class="line"> }</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="title">resolve</span>(<span class="params">newValue</span>) </span>{</span><br><span class="line"> value = newValue</span><br><span class="line"> setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> callbackList.forEach(<span class="function">(<span class="params">callback</span>)=></span>{</span><br><span class="line"> callback(value)</span><br><span class="line"> })</span><br><span class="line"> },<span class="number">0</span>)</span><br><span class="line"> }</span><br><span class="line"> fn(resolve);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p><strong>手动实现一个Promise：</strong></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><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><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Promise</span>()</span>{</span><br><span class="line"> construtor(fn){</span><br><span class="line"> <span class="comment">// 执行队列</span></span><br><span class="line"> <span class="keyword">this</span>._wathcList=[]</span><br><span class="line"> <span class="comment">// 成功</span></span><br><span class="line"> <span class="keyword">this</span>._success_res=<span class="literal">null</span></span><br><span class="line"> <span class="comment">// 失败</span></span><br><span class="line"> <span class="keyword">this</span>._error_res=<span class="literal">null</span></span><br><span class="line"> <span class="keyword">this</span>._status=<span class="string">"success"</span></span><br><span class="line"> fn(<span class="function">(<span class="params">...args</span>))=></span>{</span><br><span class="line"> <span class="comment">// 保存成功数据</span></span><br><span class="line"> <span class="keyword">this</span>._success_res=args</span><br><span class="line"> <span class="keyword">this</span>._status=<span class="string">'success'</span></span><br><span class="line"> <span class="comment">// 若为异步则回头执行then成功方法</span></span><br><span class="line"> <span class="keyword">this</span>._watchList.forEach(<span class="function"><span class="params">element</span> =></span> {</span><br><span class="line"> element.fn1(...args);</span><br><span class="line"> });</span><br><span class="line"> },(...args)=>{</span><br><span class="line"> <span class="comment">// 保存失败数据</span></span><br><span class="line"> <span class="keyword">this</span>._error_res=args</span><br><span class="line"> <span class="keyword">this</span>._status=<span class="string">'error'</span></span><br><span class="line"> <span class="comment">// 若为异步则回头执行then成功方法</span></span><br><span class="line"> <span class="keyword">this</span>._watchList.forEach(<span class="function"><span class="params">element</span> =></span> {</span><br><span class="line"> element.fn2(...args);</span><br><span class="line"> });</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// then 函数</span></span><br><span class="line"> then(fn1, fn2) {</span><br><span class="line"> <span class="keyword">if</span> (<span class="keyword">this</span>._status === <span class="string">"success"</span>) {</span><br><span class="line"> fn1(...this._success_res);</span><br><span class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (<span class="keyword">this</span>._status === <span class="string">"error"</span>) {</span><br><span class="line"> fn2(...this._error_res);</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="keyword">this</span>._watchList.push({</span><br><span class="line"> fn1,</span><br><span class="line"> fn2</span><br><span class="line"> })</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p><strong>实现Promise.all 以及 race</strong></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><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><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 实现Promise.all 以及 race</span></span><br><span class="line"><span class="built_in">Promise</span>.myall = <span class="function"><span class="keyword">function</span> (<span class="params">arr</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, reject</span>) =></span> {</span><br><span class="line"> <span class="keyword">if</span> (arr.length === <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">return</span> resolve([])</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="keyword">let</span> res = [],</span><br><span class="line"> count = <span class="number">0</span></span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i < arr.length; i++) {</span><br><span class="line"> <span class="comment">// 同时也能处理arr数组中非Promise对象</span></span><br><span class="line"> <span class="keyword">if</span> (!(arr[i] <span class="keyword">instanceof</span> <span class="built_in">Promise</span>)) {</span><br><span class="line"> res[i] = arr[i]</span><br><span class="line"> <span class="keyword">if</span> (++count === arr.length)</span><br><span class="line"> resolve(res)</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> arr[i].then(<span class="function"><span class="params">data</span> =></span> {</span><br><span class="line"> res[i] = data</span><br><span class="line"> <span class="keyword">if</span> (++count === arr.length)</span><br><span class="line"> resolve(res)</span><br><span class="line"> }, err => {</span><br><span class="line"> reject(err)</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"> })</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="built_in">Promise</span>.prototype.race = <span class="function"><span class="keyword">function</span> (<span class="params">arr</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, reject</span>) =></span> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i < arr.length; i++) {</span><br><span class="line"> <span class="comment">// 同时也能处理arr数组中非Promise对象</span></span><br><span class="line"> <span class="keyword">if</span> (!(arr[i] <span class="keyword">instanceof</span> <span class="built_in">Promise</span>)) {</span><br><span class="line"> <span class="built_in">Promise</span>.resolve(arr[i]).then(resolve, reject)</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> arr[i].then(resolve, reject)</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">}</span><br><span class="line"></span><br><span class="line"><span class="built_in">Promise</span>.prototype.finally = <span class="function"><span class="keyword">function</span> (<span class="params">callback</span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">this</span>.then(<span class="function">(<span class="params">value</span>) =></span> {</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">Promise</span>.resolve(callback()).then(<span class="function"><span class="params">()</span> =></span> {</span><br><span class="line"> <span class="keyword">return</span> value;</span><br><span class="line"> });</span><br><span class="line"> }, (err) => {</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">Promise</span>.resolve(callback()).then(<span class="function"><span class="params">()</span> =></span> {</span><br><span class="line"> <span class="keyword">throw</span> err;</span><br><span class="line"> });</span><br><span class="line"> });</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="嵌套使用"><a href="#嵌套使用" class="headerlink" title="嵌套使用"></a>嵌套使用</h3><p>Promise可以嵌套使用，这样可以是多个任务有条不紊地进行，假设p1是一个Promise对象而p2、p3都是能够产生Promise对象的方法(如果直接new那么Promise将会被直接执行)，那么你可以这样写，使得他们按照顺序执行，并且可以一次性处理他们产生的错误。</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><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> p1 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve, reject</span>) =></span> {</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="string">'p1'</span>);</span><br><span class="line"> setTimeout(<span class="function"><span class="params">()</span> =></span> {</span><br><span class="line"> resolve(<span class="string">'p2'</span>);</span><br><span class="line"> }, <span class="number">1000</span>)</span><br><span class="line">});</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> p2 = <span class="function">(<span class="params">result</span>) =></span> <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve, reject</span>) =></span> {</span><br><span class="line"> <span class="built_in">console</span>.log(result);</span><br><span class="line"> setTimeout(<span class="function"><span class="params">()</span> =></span> {</span><br><span class="line"> resolve(<span class="string">'p3'</span>);</span><br><span class="line"> }, <span class="number">2000</span>);</span><br><span class="line">});</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> p3 = <span class="function">(<span class="params">result</span>) =></span> <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve, reject</span>) =></span> {</span><br><span class="line"> <span class="built_in">console</span>.log(result);</span><br><span class="line"> setTimeout(<span class="function"><span class="params">()</span> =></span> {</span><br><span class="line"> resolve(<span class="string">'over'</span>);</span><br><span class="line"> }, <span class="number">3000</span>);</span><br><span class="line">});</span><br><span class="line"></span><br><span class="line">p1</span><br><span class="line"> .then(p2)</span><br><span class="line"> .then(p3)</span><br><span class="line"> .then(<span class="function">(<span class="params">result</span>) =></span> {</span><br><span class="line"> <span class="built_in">console</span>.log(result);</span><br><span class="line"> })</span><br><span class="line"> .catch(<span class="function">(<span class="params">error</span>) =></span> {</span><br><span class="line"> <span class="built_in">console</span>.log(error);</span><br><span class="line"> });</span><br></pre></td></tr></table></figure>
<h2 id="lite-Promise"><a href="#lite-Promise" class="headerlink" title="lite Promise"></a>lite Promise</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><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">isFunction</span>(<span class="params">target</span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">Object</span>.prototype.toString.call(target) === <span class="string">"[object Function]"</span>;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">_Promise</span> </span>{</span><br><span class="line"> <span class="keyword">constructor</span>(executor) {</span><br><span class="line"> <span class="keyword">this</span>.status = <span class="string">"pending"</span>;</span><br><span class="line"> <span class="keyword">this</span>.onResolveCallbacks = [];</span><br><span class="line"> <span class="keyword">this</span>.onRejectCallbacks = [];</span><br><span class="line"></span><br><span class="line"> <span class="keyword">let</span> handlerGenerator = <span class="function"><span class="params">status</span> =></span> <span class="function"><span class="params">data</span> =></span> {</span><br><span class="line"> <span class="keyword">this</span>.data = data;</span><br><span class="line"> <span class="keyword">this</span>.status = status;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">let</span> dict = {</span><br><span class="line"> resolve: <span class="string">"onResolveCallbacks"</span>,</span><br><span class="line"> reject: <span class="string">"onRejectCallbacks"</span></span><br><span class="line"> };</span><br><span class="line"></span><br><span class="line"> executeCallback(<span class="keyword">this</span>[dict[status]]);</span><br><span class="line"> };</span><br><span class="line"></span><br><span class="line"> <span class="keyword">let</span> executeCallback = <span class="function"><span class="params">cbs</span> =></span> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">let</span> cb <span class="keyword">of</span> cbs) {</span><br><span class="line"> cb(<span class="keyword">this</span>.data);</span><br><span class="line"> }</span><br><span class="line"> };</span><br><span class="line"></span><br><span class="line"> <span class="keyword">let</span> resolve = handlerGenerator(<span class="string">"resolve"</span>).bind(<span class="keyword">this</span>);</span><br><span class="line"> <span class="keyword">let</span> reject = handlerGenerator(<span class="string">"reject"</span>).bind(<span class="keyword">this</span>);</span><br><span class="line"></span><br><span class="line"> executor(resolve, reject);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> _funcPoly(data) {</span><br><span class="line"> <span class="keyword">return</span> data;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> _PromiseGenerator(onResolved, onRejected) {</span><br><span class="line"> <span class="keyword">let</span> self = <span class="keyword">this</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> _Promise(<span class="function">(<span class="params">resolve, reject</span>) =></span> {</span><br><span class="line"> <span class="keyword">let</span> body = <span class="function"><span class="params">()</span> =></span> {</span><br><span class="line"> <span class="keyword">let</span> status = self.status;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">let</span> result =</span><br><span class="line"> status === <span class="string">"resolve"</span></span><br><span class="line"> ? onResolved(self.data)</span><br><span class="line"> : status === <span class="string">"reject"</span></span><br><span class="line"> ? onRejected(self.data)</span><br><span class="line"> : <span class="literal">undefined</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (result <span class="keyword">instanceof</span> _Promise) {</span><br><span class="line"> <span class="keyword">return</span> resolvedResult.then(resolve, reject);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (status === <span class="string">"resolve"</span>) {</span><br><span class="line"> resolve(result);</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">catch</span> (e) {</span><br><span class="line"> reject(e);</span><br><span class="line"> }</span><br><span class="line"> };</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (self.status === <span class="string">"pending"</span>) {</span><br><span class="line"> self.onResolveCallbacks.push(body);</span><br><span class="line"> self.onRejectCallbacks.push(body);</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> body(self.status);</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"> then(onResolved = <span class="keyword">this</span>._funcPoly, onRejected = <span class="keyword">this</span>._funcPoly) {</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">this</span>._PromiseGenerator(onResolved, onRejected);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">catch</span>(onRejected) {</span><br><span class="line"> <span class="keyword">this</span>.then(<span class="literal">null</span>, onRejected);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">new</span> _Promise(<span class="function"><span class="keyword">function</span>(<span class="params">resolve</span>)</span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="number">1</span>)</span><br><span class="line"> resolve()</span><br><span class="line">}).then(<span class="built_in">console</span>.log(<span class="number">2</span>))</span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">3</span>)</span><br></pre></td></tr></table></figure>
<h2 id="如何取消-promise"><a href="#如何取消-promise" class="headerlink" title="如何取消 promise"></a>如何取消 <code>promise</code></h2><ol>
<li><p>Promise.race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><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></pre></td><td class="code"><pre><span class="line"><span class="comment">// 方法一 取消promise方法 promise.race方法</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">wrap</span>(<span class="params">p</span>) </span>{</span><br><span class="line"> <span class="keyword">let</span> obj = {};</span><br><span class="line"> <span class="keyword">let</span> p1 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve, reject</span>) =></span> {</span><br><span class="line"> obj.resolve = resolve;</span><br><span class="line"> obj.reject = reject;</span><br><span class="line"> });</span><br><span class="line"> obj.promise = <span class="built_in">Promise</span>.race([p1, p]);</span><br><span class="line"> <span class="keyword">return</span> obj;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> promise = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve, reject</span>) =></span> {</span><br><span class="line"> setTimeout(<span class="function"><span class="params">()</span> =></span> {</span><br><span class="line"> resolve(<span class="number">123</span>);</span><br><span class="line"> }, <span class="number">1000</span>);</span><br><span class="line">});</span><br><span class="line"><span class="keyword">let</span> obj = wrap(promise);</span><br><span class="line">obj.promise.then(<span class="function"><span class="params">res</span> =></span> {</span><br><span class="line"> <span class="built_in">console</span>.log(res);</span><br><span class="line">});</span><br><span class="line">obj.resolve(<span class="string">"请求被拦截了"</span>);</span><br><span class="line"></span><br><span class="line">obj.reject(<span class="string">"请求被拒绝了"</span>);</span><br></pre></td></tr></table></figure>
</li>
<li><p>新包装一个可操控的promise</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><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">wrap</span>(<span class="params">p</span>) </span>{</span><br><span class="line"> <span class="keyword">let</span> res = <span class="literal">null</span>;</span><br><span class="line"> <span class="keyword">let</span> abort = <span class="literal">null</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">let</span> p1 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve, reject</span>) =></span> {</span><br><span class="line"> res = resolve;</span><br><span class="line"> abort = reject;</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> p1.abort = abort;</span><br><span class="line"> p.then(res, abort);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> p1;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> promise = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve, reject</span>) =></span> {</span><br><span class="line"> setTimeout(<span class="function"><span class="params">()</span> =></span> {</span><br><span class="line"> resolve(<span class="number">123</span>);</span><br><span class="line"> }, <span class="number">1000</span>);</span><br><span class="line">});</span><br><span class="line"><span class="keyword">let</span> obj = wrap(promise);</span><br><span class="line">obj.then(<span class="function"><span class="params">res</span> =></span> {</span><br><span class="line"> <span class="built_in">console</span>.log(res);</span><br><span class="line">});</span><br><span class="line">obj.abort(<span class="string">"请求被拦截"</span>);</span><br></pre></td></tr></table></figure>
</li>
</ol>
<h2 id="Promise使用注意点"><a href="#Promise使用注意点" class="headerlink" title="Promise使用注意点"></a>Promise使用注意点</h2><ol>
<li><p>一般来说，调用<code>resolve</code>或<code>reject</code>以后，<code>Promise</code>的使命就完成了，后继操作应该放到<code>then</code>方法里面，而不应该直接写在<code>resolve</code>或<code>reject</code>的后面。所以，最好在它们前面加上<code>return</code>语句，这样就不会有意外。</p>
</li>
<li><p><code>Promise</code>实例具有<code>then</code>方法，也就是说，<code>then</code>方法是定义在原型对象<code>Promise.prototype</code>上的。它的作用是为<code>Promise</code>实例添加状态改变时的回调函数。前面说过，<code>then</code>方法的第一个参数是<code>resolved</code>状态的回调函数，第二个参数（可选）是<code>rejected</code>状态的回调函数。<code>then</code>方法返回的是一个新的<code>Promise</code>实例（注意，不是原来那个<code>Promise</code>实例）。因此可以采用链式写法，即then方法后面再调用另一个<code>then</code>方法。</p>
</li>
<li><p><code>Promise.prototype.catch</code>方法是.then(null, rejection)的别名，用于指定发生错误时的回调函数。<code>getJSON</code>方法返回一个<code>Promise</code>对象，如果该对象状态变为<code>resolved</code>，则会调用<code>then</code>方法指定的回调函数；如果异步操作抛出错误，状态就会变为<code>rejected</code>，就会调用<code>catch</code>方法指定的回调函数，处理这个错误。另外，then方法指定的回调函数，如果运行中抛出错误，也会被<code>catch</code>方法捕获。</p>
</li>
<li><p>一般来说，不要在then方法里面定义<code>reject</code>状态的回调函数（即then的第二个参数），总是使用<code>catch</code>方法。</p>
</li>
<li><p>跟传统的<code>try/catch</code>代码块不同的是，如果没有使用<code>catch</code>方法指定错误处理的回调函数，<code>Promise</code>对象抛出的错误不会传递到外层代码，即不会有任何反应，<code>Promise</code>会吃掉错误。</p>
</li>
<li><p>Promise 构造函数是同步执行，then方法是异步执行。<code>.then</code> 或者 <code>.catch</code> 的参数期望是函数，传入非函数则会发生值透传。</p>
</li>
<li><p>Promise.all中如果有一个抛出异常了会如何处理?</p>
<blockquote>
<p>all和race传入的数组中如果有会抛出异常的异步任务，那么只有最先抛出的错误会被捕获，并且是被then的第二个参数或者后面的catch捕获；但并不会影响数组中
其它的异步任务的执行。</p>
</blockquote>
</li>
<li><p>promise.all并发还是串行
并发的。不过Promise.all().then()结果中数组的顺序和Promise.all()接收到的数组顺序一致。</p>
</li>
<li><p>promise.all失败获取所有请求</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><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">getData</span>(<span class="params">api</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,reject</span>) =></span> {</span><br><span class="line"> setTimeout(<span class="function"><span class="params">()</span> =></span> {</span><br><span class="line"> <span class="keyword">var</span> ok = <span class="built_in">Math</span>.random() > <span class="number">0.5</span> <span class="comment">// 模拟请求成功或失败</span></span><br><span class="line"> <span class="keyword">if</span>(ok)</span><br><span class="line"> resolve(<span class="string">'get '</span> + api + <span class="string">' data'</span>)</span><br><span class="line"> <span class="keyword">else</span>{</span><br><span class="line"> <span class="comment">// reject(api + ' fail') // 如果调用reject就会使Promise.all()进行失败回调</span></span><br><span class="line"> resolve(<span class="string">'error'</span>) <span class="comment">// Promise all的时候做判断 如果是error则说明这条请求失败</span></span><br><span class="line"> }</span><br><span class="line"> },<span class="number">2000</span>)</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="title">getDatas</span>(<span class="params">arr</span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> promises = arr.map(<span class="function"><span class="params">item</span> =></span> getData(item))</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">Promise</span>.all(promises).then(<span class="function"><span class="params">values</span> =></span> {</span><br><span class="line"> values.map(<span class="function">(<span class="params">v,index</span>) =></span> {</span><br><span class="line"> <span class="keyword">if</span>(v == <span class="string">'error'</span>){</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="string">'第'</span> + (index+<span class="number">1</span>) + <span class="string">'个请求失败'</span>)</span><br><span class="line"> }<span class="keyword">else</span>{</span><br><span class="line"> <span class="built_in">console</span>.log(v)</span><br><span class="line"> }</span><br><span class="line"> })</span><br><span class="line"> }).catch(<span class="function"><span class="params">error</span> =></span> {</span><br><span class="line"> <span class="built_in">console</span>.log(error)</span><br><span class="line"> })</span><br><span class="line">}</span><br><span class="line">getDatas([<span class="string">'./api1'</span>,<span class="string">'./api2'</span>,<span class="string">'./api3'</span>,<span class="string">'./api4'</span>]).then(<span class="function"><span class="params">()</span> =></span> <span class="string">'请求结束'</span>)</span><br></pre></td></tr></table></figure>
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<p><span>本文标题:</span><a href="/createPromise.html">Promise诞生记</a></p>
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<p><span>发布时间:</span>2018年06月05日 - 01:06</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="#Promise-概念-JS的-Promise是未来事件的表示"><span class="nav-number">1.</span> <span class="nav-text">Promise 概念(JS的 Promise是未来事件的表示)</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#定义-Promise-类型"><span class="nav-number">2.</span> <span class="nav-text">定义 Promise 类型</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#实现-resolve-和-then"><span class="nav-number">2.1.</span> <span class="nav-text">实现 resolve 和 then</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#改进1：延时resolve，修改-callback-为异步"><span class="nav-number">2.2.</span> <span class="nav-text">改进1：延时resolve，修改 callback 为异步</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#改进2：注册多个回调函数，并实现then的链式调用"><span class="nav-number">2.3.</span> <span class="nav-text">改进2：注册多个回调函数，并实现then的链式调用</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#改进3：引入状态"><span class="nav-number">2.4.</span> <span class="nav-text">改进3：引入状态</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#嵌套使用"><span class="nav-number">2.5.</span> <span class="nav-text">嵌套使用</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#lite-Promise"><span class="nav-number">3.</span> <span class="nav-text">lite Promise</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#如何取消-promise"><span class="nav-number">4.</span> <span class="nav-text">如何取消 promise</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#Promise使用注意点"><span class="nav-number">5.</span> <span class="nav-text">Promise使用注意点</span></a></li></ol></div>
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<span class="author" itemprop="copyrightHolder">陈宇（cosyer）</span>