为什么 Promise.all 的错误？：TypeError：无法读取未定义的属性“Symbol（Symbol.iterator）”

Question

I have the following controller method:

const org = await organisation.toJSON().name;
// The next line works fine, that's not the problem
const users = await organisation.getUsers(organisation.toJSON().id, User);
await Promise.all(users.forEach(async user => {
  await sendAccountEmail(
    user.email,
    user.surname,
    org
  );
}));

For the Promise line I get an error:

UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined

Any idea what is wrong with my code?

Answer 1

You're using Promise.all incorrectly. That method expects you to give it an array (or another iterable object) filled with Promises, and then it waits until all Promises in that array have settled (resolved or rejected). You shouldn't be iterating over the array inside the parameter, especially not with forEach which returns undefined, thus making your code equivalent to await Promise.all(undefined) -- so you can see now why that would error, can't you?

What you should be doing is using Array.map to convert your array of users into an array of Promises that you want to wait for:

const userPromises = users.map(user => sendAccountEmail(user.email, user.surname, org));
await Promise.all(userPromises);

Answer 2

Though you already have your answer, I thought I'd explain the actual error you're getting because it's useful to understand what this specific error means and it goes to how Promise.all() actually works.

The error you're getting is this:

UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined

There are two separate things going on here.

1. Promise.all() expects an iterable as its argument, something that it can get an iterator from and iterate the items in it. Your code is passing the result of .forEach() which is undefined , so the first part of this error is that undefined is not an iterable and the way this is discovered is that it doesn't have a property Symbol(Symbol.iterator) .

2. The second part of this error is the UnhandlePromiseRejectionWarning . This shows that await Promise.all(...) rejects and you have no error handling for that. You should either have a try/catch around the await or be using .catch() as in Promise.all().catch() . The answer you have accepted does not address this coding problem. While fixing your code stops the rejection from happening in normal execution of your code, sendAccountEmail() sounds to me like a function that could run into a failure and could reject. Your code needs to handle that case. We can't really suggest the best overall error handling strategy here because we'd need to see the larger context of the code to know what is supposed to happen when errors occur. But, you need to handle all possible errors.