Javascript 如何在使用FOR AWAIT时从异步生成器中获取返回值

发布于12月28日

我有以下代码:

for await (const part of resolveData(data)) {
    console.log({part});
}

它遍历如下所示的异步生成器:

const resolveData = async function* <T> (data: SomeValue<T>) {
    if (<expression1>) {
        if (<expression2>) {
            console.log("valid");
            yield 1;
        }
    }
    return 2;
}

For aWait循环的输出如下所示:

{ part: 1 }

我还希望获得返回值(在本例中为2).如何在循环内部或外部获取值？

已try 在网上查找，但一无所获.

推荐答案

当您在生成器或BLOG生成器中使用return时，它会在结果对象上设置value的值，以表示done为true.但是for-of和for-await-of并不运行结果对象的循环体，其中done为true.下面是一个简单的例子:

<script type="module">
// Note: Using an async iterator even though we never await
// because the OP's code is async
async function* example() {
    yield 1;
    return 2;
}

for await (const x of example()) {
    console.log(x);
}
</script>

(Sadly, the "JavaScript" panel in Stack Snippets can't be run as a module, and I wanted to use top-level 100, so I had to put the JavaScript in the "HTML" panel instead.)

请注意，只显示了1个.(for-await-of也是如此.)

您至少有两个 Select :

1.将return 2更改为yield 2，这样生成器在生成2的时候还不会"完成"，如下所示:

<script type="module">
// Note: Using an async iterator even though we never await
// because the OP's code is async
async function* example() {
    yield 1;
    yield 2; // `yield` instead of `return`
}

for await (const x of example()) {
    console.log(x);
}
</script>

2.更改代码以直接使用迭代器，而不是使用for-await-of.

<script type="module">
   // Note: Using an async iterator even though we never await
   // because the OP's code is async
   async function* example() {
       yield 1;
       return 2;
   }
   
   const it = example();
   let r;
   while (!(r = await it.next()).done) {
       console.log(r.value);
   }
   console.log(r.value);
   </script>

在这两种情况下，您现在看到的是1和2.