blocks|key|1026452|text|下面的函数不返回任何内容。它也不是一个承诺，它将用一个价值来解决。它只是一个函数，它为一个函数设置一个定时器，这个函数(将在窗口的上下文中运行)并返回1。返回的值在任何地方都不会被捕获。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1026453|async+function+test(){
++setTimeout(function(){return+1},+100)
};|code-block|syntax|javascript|1026454|要从setTimeout访问值，您必须将其包装在一个承诺中，并让允诺与该值一起解析。|1026455|​|1026456|async+function+test(){
++return+new+Promise((resolve,reject)+=>+setTimeout(function(){resolve(1);},+100));
};
test().then(console.log);


async+function+test2(){
++return+1;
};
test2().then(console.log);+//+Displayed|1026457|1026458|另一种不使用承诺访问setTimeout内部值的方法是传递回调，如下所示。|1026459|1026460|async+function+test(callback){
++setTimeout(function(){callback(1)},+100)
};
test(console.log);|1026461|1026462|上面的代码说明了为什么使用承诺更容易对代码进行推理。您想要访问值的任何代码段(从setTimeout内部)都必须在回调中。这会变得很混乱|1026463|entityMap^0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Z|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|10|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|M|3|J|5|6|7|12|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|13|8|@]|9|@]|A|$]]|$1|P|3|J|5|6|7|14|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|S|3|J|5|6|7|16|8|@]|9|@]|A|$]]|$1|T|3|U|5|6|7|17|8|@]|9|@]|A|$]]|$1|V|3|-4|5|6|7|18|8|@]|9|@]|A|$]]]|W|$]]

<p>The below function does not return anything. Neither is it a promise, that it would resolve with a value. It is just a function that sets a timer for a function that (will run in window's context) and return 1. The returned value is not captured anywhere.</p>
<pre><code>async function test(){
  setTimeout(function(){return 1}, 100)
};
</code></pre>
<p>To access the value from setTimeout you will have to wrap it in a promise and let the promise resolve with that value.</p>
<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>async function test(){
  return new Promise((resolve,reject) =&gt; setTimeout(function(){resolve(1);}, 100));
};
test().then(console.log);


async function test2(){
  return 1;
};
test2().then(console.log); // Displayed</code></pre>
</div>
</div>
</p>
<p>Another way to access value inside setTimeout, without using promises is to pass callbacks like below.</p>
<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>async function test(callback){
  setTimeout(function(){callback(1)}, 100)
};
test(console.log);</code></pre>
</div>
</div>
</p>
<p>The above code shows why it is easier to reason about code with promises. Any piece of code, which you want to have access to the value (from inside setTimeout) will have to be in the callback. This gets messy</p>


blocks|key|1026495|text|首先，您忘记了test函数中的返回语句。如果没有返回语句，该函数将返回一个解析为Promise的undefined。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1026496|第二，setTimeout不返回承诺。相反，您必须显式地创建它。|1026497|应该是这样的。|1026498|async+function+test()+{
++return+new+Promise((resolve)+=>+setTimeout(()+=>+resolve(1),+100))
}|code-block|syntax|javascript|1026499|但是您的代码等价于以下内容，因为您没有以任何方式使用setTimeout。|1026500|async+function+test()+{
++return+undefined
}|1026501|创建异步函数有两个选项。历史上，第一个函数接受回调。第二个选项是从函数返回一个承诺。|1026502|许多库函数接受回调，并在发生异步事件时调用回调。这样一个函数的一个例子是setTimeout。|1026503|要使用接受await回调的函数，可以将其包装在一个新的承诺中。|1026504|例如，我们有一个库函数libFunc(...,+callback)，其中callback是一个接受一些参数的函数。|1026505|你可以像这样创造新的承诺。|1026506|const+result+=+await+new+Promise(
++(resolve)+=>+libFunc(/*some+arguments*/,
++++(callbackArg)+=>+resolve(callbackArg)))|1026507|你可以查看我关于JavaScript中回调的异步函数主题的帖子|1026508|entityMap|0|LINK|mutability|MUTABLE|url|https://victorsmirnov.blog/asynchronous-functions-from/^0|7|4|14|7|1C|9|0|3|A|0|0|0|Q|A|0|0|0|10|A|0|5|5|0|B|M|10|8|0|0|0|8|I|0|0^^$0|@$1|2|3|4|5|6|7|1E|8|@$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]|$9|1J|A|1K|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1L|8|@$9|1M|A|1N|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|1O|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|1P|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|1Q|8|@$9|1R|A|1S|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|L|7|1T|8|@]|D|@]|E|$M|N]]|$1|S|3|T|5|6|7|1U|8|@]|D|@]|E|$]]|$1|U|3|V|5|6|7|1V|8|@$9|1W|A|1X|B|C]]|D|@]|E|$]]|$1|W|3|X|5|6|7|1Y|8|@$9|1Z|A|20|B|C]]|D|@]|E|$]]|$1|Y|3|Z|5|6|7|21|8|@$9|22|A|23|B|C]|$9|24|A|25|B|C]]|D|@]|E|$]]|$1|10|3|11|5|6|7|26|8|@]|D|@]|E|$]]|$1|12|3|13|5|L|7|27|8|@]|D|@]|E|$M|N]]|$1|14|3|15|5|6|7|28|8|@]|D|@$9|29|A|2A|1|2B]]|E|$]]|$1|16|3|-4|5|6|7|2C|8|@]|D|@]|E|$]]]|17|$18|$5|19|1A|1B|E|$1C|1D]]]]

<p>First, you forgot the return statement in the <code>test</code> function. Without a return statement, the function returns a <code>Promise</code> which resolves to <code>undefined</code>.</p>
<p>Second, <code>setTimeout</code> does not return a promise. Instead, you have to create it explicitly.</p>
<p>It should be something like this.</p>
<pre class="lang-js prettyprint-override"><code>async function test() {
  return new Promise((resolve) =&gt; setTimeout(() =&gt; resolve(1), 100))
}

</code></pre>
<p>But your code is equivalent to the following because you do not use <code>setTimeout</code> in any way.</p>
<pre class="lang-js prettyprint-override"><code>async function test() {
  return undefined
}
</code></pre>
<p>There are two options to create an async function. Historically the first one is that function accepts a callback. The second option is to return a promise from the function.</p>
<p>Many library functions accept a callback and call the callback when the async event happens. An example of such a function is the <code>setTimeout</code>.</p>
<p>To use a function which accepts a callback with <code>await</code>, you can wrap it in a new Promise.</p>
<p>For example, we have a library function <code>libFunc(..., callback)</code> where <code>callback</code> is a function which accepts some arguments.</p>
<p>You can create a new promise like this.</p>
<pre class="lang-js prettyprint-override"><code>const result = await new Promise(
  (resolve) =&gt; libFunc(/*some arguments*/,
    (callbackArg) =&gt; resolve(callbackArg)))

</code></pre>
<p>You can check my post on the topic <a href="https://victorsmirnov.blog/asynchronous-functions-from/" rel="nofollow noreferrer">Async function from callback in JavaScript</a></p>


<pre><code>async function test(){
  setTimeout(function(){return 1}, 100)
};
test().then(console.log); // Never displayed
</code></pre>
<p>If the setTimeout wrap is removed, it works:</p>
<pre><code>async function test(){
  return 1;
};
test().then(console.log); // Displayed
</code></pre>


Why this promise is never resolved? (Simple JavaScript in Chrome)

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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async function test(){  setTimeout(function(){return 1}, 100)};test().then(console.log); // Never displayed如果删除了setTimeout包装，它可以工作：async function test(){  retur...

问为什么这个承诺从来没有实现过？(Chrome中的简单JavaScript )
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问为什么这个承诺从来没有实现过？(Chrome中的简单JavaScript )EN