Add a Timeout to a Promise in JavaScript (Exercise)

The challenge

Write withTimeout(promise, ms) that resolves with the promise’s value if it finishes within ms milliseconds, or rejects with new Error("Timed out") if it takes longer. The wait(ms, value) helper is there to help you test.

Try it

Write your answer below, then press Check my answer. Your code runs right here in your browser, and the tests tell you what works.

Your solution

It prints

What the tests check

withTimeout(wait(30, "fast"), 200) gives "fast"
withTimeout(wait(300, "slow"), 50).catch((e) => e.message) gives "Timed out"

Hints

Stuck? Open one hint at a time. Try again after each one.

Hint 1

Promise.race([a, b]) settles as soon as the first one does.

Hint 2

Make a second promise that rejects after ms, and race them.

A solution

There’s always more than one right way. Read it, make sure you understand it, then try writing it yourself without looking.

Show one possible solution

Solution

const wait = (ms, value) => new Promise((resolve) => setTimeout(() => resolve(value), ms));

function withTimeout(promise, ms) {
  const timer = new Promise((_, reject) => {
    setTimeout(() => reject(new Error("Timed out")), ms);
  });
  return Promise.race([promise, timer]);
}

Learn what you need

This exercise uses what Freshman Year: Asynchronous JavaScript: Waiting Without Freezing teaches. Read those notes first if any of it is new.

Add a timeout to a promise