不能从异步承诺执行程序函数中抛出错误 [英] Can't throw error from within an async promise executor function

问题描述

我一直试图理解为什么下面的代码没有捕获 throw 。如果从新Promise中删除 async 关键字（async（resolve，... 部分），那么它可以正常工作，因此它与Promise执行程序是异步函数有关。

I've been trying to get a conceptual understanding of why the following code doesn't catch the throw. If you remove the async keyword from the new Promise(async (resolve, ... part then it works fine, so it has to do with the fact that the Promise executor is an async function.

(async function() {

  try {
    await fn();
  } catch(e) {
    console.log("CAUGHT fn error -->",e)
  }

})();

function fn() {

  return new Promise(async (resolve, reject) => {
    // ...
    throw new Error("<<fn error>>");
    // ...
  });

}

答案此处，这里，这里重复如果你'回覆 在任何其他异步回调中，您必须使用 reject ，但是异步它们不是指 async 函数，所以我不认为他们的解释适用于此（如果他们这样做，我不明白如何）。

The answers here, here, and here repeat that "if you're in any other asynchronous callback, you must use reject", but by "asynchronous" they're not referring to async functions, so I don't think their explanations apply here (and if they do, I don't understand how).

如果不是抛出我们使用拒绝，上面的代码工作正常。我想明白，从根本上，为什么 throw 在这里不起作用。谢谢！

If instead of throw we use reject, the above code works fine. I'd like to understand, fundamentally, why throw doesn't work here. Thanks!

推荐答案

这是 承诺构造函数反模式！

This is the async/await version of the Promise constructor antipattern!

永远不要 曾经使用异步函数作为 Promise 执行函数_{（即使你能使它工作 ¹ ）}！

Never ever use an async function as a Promise executor function _{(even when you can make it work¹)}!

_{[1：通过致电解决和拒绝而不是使用返回和抛出语句]}

_{[1: by calling resolve and reject instead of using return and throw statements]}

byasynchronous他们不是指 async 函数，所以我不认为他们的解释适用于此处

by "asynchronous" they're not referring to async functions, so I don't think their explanations apply here

他们也可以。 无法工作的简单示例是

They could as well. A simple example where it cannot work is

new Promise(async function() {
    await delay(…);
    throw new Error(…);
})

相当于

new Promise(function() {
    return delay(…).then(function() {
        throw new Error(…);
    });
})

现在很清楚 throw 是在异步回调中。

where it's clear now that the throw is inside an asynchronous callback.

< a href =https://stackoverflow.com/a/28692824/1048572> Promise 构造函数只能捕获同步异常，以及 async函数 永不抛出 - 它总是返回一个promise（虽然可能会被拒绝）。并且该返回值被忽略，因为承诺正在等待 resolve 被调用。

The Promise constructor can only catch synchronous exceptions, and an async function never throws - it always returns a promise (which might get rejected though). And that return value is ignored, as the promise is waiting for resolve to be called.

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