Advanced

Promises

An object representing the future result of an async operation.

1. Explanation

A Promise is in one of three states: pending, fulfilled, or rejected. Use .then(onFulfilled) and .catch(onRejected) to handle the outcome. Create one with new Promise((resolve, reject) => ...). Modern code prefers async/await on top of Promises.

2. Example

Try it yourself

const p = new Promise((resolve, reject) => {
  const ok = true;
  setTimeout(() => ok ? resolve("done") : reject("failed"), 500);
});

p.then(v => console.log("✅", v))
 .catch(e => console.log("❌", e))
 .finally(() => console.log("settled"));

Console

Click Run to see output.

3. Expected output

output

✅ done
settled

4. Common mistakes

✗Forgetting to return the inner promise inside .then — breaks chaining
✗Not adding a .catch — unhandled promise rejections crash modern Node
✗Mixing Promises and callbacks for the same API

5. Practice questions

Wrap setTimeout in a promise-returning 'wait(ms)' helper
Use Promise.all to wait for three timers
Chain three .then steps that each transform the value

→ Try more practice problems