JavaScript Interview Question: Promises vs async/await

Short answer: a promise represents a value that will be available later; it's pending, then fulfilled or rejected. async/await is syntax on top of promises: an async function always returns a promise, and await pauses the function until a promise settles.

Same logic, two styles

// Promise chain
function loadUser(id) {
  return fetch(`/api/users/${id}`)
    .then((res) => res.json())
    .catch((err) => { console.error(err); return null; });
}

// async/await
async function loadUser(id) {
  try {
    const res = await fetch(`/api/users/${id}`);
    return await res.json();
  } catch (err) {
    console.error(err);
    return null;
  }
}

Key facts interviewers look for

• An async function returns a promise even if you return 5.
• throw inside an async function rejects its promise.
• await only pauses that function, not the whole program.
• Unhandled rejections are errors; always catch somewhere.

Sequential vs parallel

// Sequential: ~2 seconds if each takes 1 second
const a = await getA();
const b = await getB();

// Parallel: ~1 second
const [a, b] = await Promise.all([getA(), getB()]);

Common mistakes

• array.forEach(async (x) => await save(x)) doesn't wait for anything. Use for...of with await, or Promise.all(array.map(...)).
• Forgetting await, so you get a promise instead of a value (and errors escape your try/catch).
• Awaiting independent calls one by one when they could run in parallel.