Returns the first promise that resolves with a truthy value, or undefined if all promises resolve with a nullish value.

promise-race-truthy.js

const NEVER = new Promise(() => {});

async function raceTruthy(iterable) {
  const ps = [...iterable].map(_ => Promise.resolve(_));
  let { length } = ps;
  const continueWhenNullish = value => value != null
    ? value
    : --length > 0
      ? NEVER
      : undefined;
  return Promise.race(ps.map(p => p.then(continueWhenNullish)));
}

Returns the first promise that resolves with a truthy value, or undefined if all promises resolve with a nullish value.

Note that this inherits the behavior of Promise.race, where the returned promise rejects as soon as one input promise rejects.

Example use case: Getting a cached response from a number of caches, without resorting to a lookup in all caches via caches.match:

const matching = await raceTruthy([
  caches.open(SHELL_CACHE).then(c => c.match(url)),
  caches.open(ASSETS_CACHE).then(c => c.match(url)),
  caches.open(CONTENT_CACHE).then(c => c.match(url)),
]);