未处理的 promise 拒绝与 Promise.all 解决并尝试/捕获

Question

My idea is as follows: I want to send multiple requests simultaneously without having to wait until priors execution.

So my pseudo code looks as follows:

 function sleep(ms) { return new Promise(resolve => setTimeout(resolve, ms)); } function failingRequest(){ return new Promise((resolve, reject) => { reject('Request failed'); }); } function successRequest(){ return new Promise((resolve, reject) => { resolve('Request success'); }); } async function main() { try { let executions = []; executions.push(failingRequest()); await sleep(4000); executions.push(successRequest()); let result = await Promise.allSettled(executions); console.log(result); } catch (err) { console.log('Outer error occured.'); console.log(err.message); } console.log('done'); } main();

Running this code here works as intended in the browser but gives me following output running with node:

node:761) UnhandledPromiseRejectionWarning: Request failed
api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:761) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exi not handled will terminate the Node.js process with a non-zero exit code.
[
  { status: 'rejected', reason: 'Request failed' },
  { status: 'fulfilled', value: 'Request success' }
]
done
(node:761) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 1)

Any idea why this is happening?

Note that I only inserted sleep so I could test if the catch block will be executed in case the first request fails WHICH WOULD NOT BE THE DESIRED BEHAVIOR . I want to initiate those requests all at the same time and I do not care if one fails. I want to check later with let result = await Promise.allSettled(executions); which requests worked and which failed. I hope this is clear.

Answer 1

Interesting question - the problem is that you're not actually simulating async requests. In fact your two request-methods are simply creating promises which are resolved/rejected synchronously/immediately. You would need to put await before failingRequest() in order for the rejected promise to be caught in the surrounding try/catch but this is probably not what you want.

Instead you should not "start" the promises immediately, it should rather be something like:

try {
        let executions = [];
        executions.push(failingRequest);
        await sleep(4000);
        executions.push(successRequest);
        let result = await Promise.allSettled(executions.map(promiseFn => promiseFn()));
        console.log(result);
    } catch (err) {
        console.log('Outer error occured.');
        console.log(err.message);
    }

This will log

[
  { status: 'rejected', reason: 'Request failed' },
  { status: 'fulfilled', value: 'Request success' }
]
done

as expected.

Answer 2

Any idea why this is happening?

You create the failingRequest() and then wait 4 seconds before handling it.

I only inserted sleep so I could test

… and thereby you caused the unhandled rejection. Remove the await sleep(4000); and it'll work as expected!