Filter rejected Promises

Question

I have an array of Promise objects where I want to map on and filter the rejected Promises.

Expected output:

const promises = [
  failedPromise,
  successPromise,
  successPromise,
];

const resolvedPromises = promises.map(promise => ???);

The resolvedPromises variable should container the output of the two successPromise promises.

How would I implement such algorithm?

Answer 1

You can't inspect standard promises in a synchronous way, so you cannot filter the array by their resolution status. But you can employ Promise.all to get a list of the resolved values. Here is a little trick how to handle rejections:

const promises = [
  Promise.resolve(1),
  Promise.reject(2),
  Promise.resolve(3)
];

const FAIL_TOKEN = {};

const resolvedPromises = Promise.all(
  promises.map(p => p.catch(e => FAIL_TOKEN))
).then(
  values => values.filter(v => v !== FAIL_TOKEN)
);

resolvedPromises.then(values => console.log(values));