The four combinators in JavaScript

all, allSettled, race and any differ in what counts as finished.

const quick = (value, ms) =>
  new Promise((resolve) => setTimeout(() => resolve(value), ms));
const failing = (ms) =>
  new Promise((_, reject) => setTimeout(() => reject(new Error("boom")), ms));

async function main() {
  console.log(await Promise.all([quick("a", 5), quick("b", 1)]));

  const settled = await Promise.allSettled([quick("ok", 1), failing(2)]);
  console.log(settled.map((r) => r.status));

  console.log(await Promise.race([quick("fast", 1), quick("slow", 20)]));
  console.log(await Promise.any([failing(1), quick("survivor", 5)]));

  try {
    await Promise.all([quick("a", 1), failing(2)]);
  } catch (error) {
    console.log("all rejected:", error.message);
  }
}

main();

How it works

1. all rejects on the first failure; allSettled never rejects.
2. race settles with whichever settles first, success or not.
3. any waits for the first success, rejecting only if all fail.

Keywords and builtins used here

• Promise
• async
• await
• catch
• const
• function
• try

The run, in numbers

Lines

22

Characters to type

670

Tokens

235

Three-star pace

95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 148 seconds.

Type this snippet

Step 5 of 5 in Promises, step 5 of 19 in Promises & async.

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