为什么这段代码会触发@typescript-eslint/promise-function-async？

Question

I have this code in my little project:

close(flush = false): void {
    if (this.ws?.CLOSING || this.ws?.CLOSED) {
        return;
    }

    if (flush) {
        // I really don't know how to fix that
        // eslint-disable-next-line @typescript-eslint/promise-function-async
        const sendPromises = this.state.queue.map((message) =>
            this.sendAsync(message)
        );
        void Promise.all(sendPromises).then(() => this.ws?.close());
    } else {
        this.ws?.close();
    }
}

When I run xo (which uses typescript-eslint) on it, @typescript-eslint/promise-function-async fails. I made a few changes, but it still fails. Can anyone give me an explanation about why this doesn't work?

What I tried:

// first
const sendPromises: Promise<void> = this.state.queue.map((message) => this.sendAsync(message));
// second
const sendPromises = this.state.queue.map((message): Promise<void> => this.sendAsync(message));

Answer 1

Here is the description of the @typescript-eslint/promise-function-async rule:

Requires any function or method that returns a Promise to be marked async.

With example for incorrect code:

const arrowFunctionReturnsPromise = () => Promise.resolve('value');

function functionReturnsPromise() {
  return Promise.resolve('value');
}

and correct code:

const arrowFunctionReturnsPromise = async () => Promise.resolve('value');

async function functionReturnsPromise() {
  return Promise.resolve('value');
}

Your code fails the first line of the example. More specifically the problem is here:

const sendPromises = this.state.queue.map((message) =>
  this.sendAsync(message)
);

You're calling .map() with an arrow function that produces a promise, so according to the rule, you have to mark it as async like so:

const sendPromises = this.state.queue.map(async (message) =>
//                                        ^^^^^
  this.sendAsync(message)
);

Answer 2

After I read the comments I noticed that I should add async to my arrow function.

Ironically, I fixed the same error before but I overlooked this one.

Thanks for helping.