Promises and async/await without the confusion

A promise stands for a value that will arrive later: a server response, a file, a timer. It is pending at first, then either fulfilled with a value or rejected with an error.

async and await

An async function always returns a promise. Inside it, await pauses that function (not the whole program) until the promise settles.

const wait = (ms, value) => new Promise(resolve => setTimeout(() => resolve(value), ms));
const fail = ms => new Promise((_, reject) => setTimeout(() => reject(new Error('server down')), ms));
async function loadProfile() {
  const name = await wait(30, 'Asha');
  const branch = await wait(30, 'CSE');
  return `${name} (${branch})`;
}
async function main() {
  console.log(await loadProfile());
  try { await fail(10); } catch (e) { console.log('caught:', e.message); }
  const start = Date.now();
  const [a, b] = await Promise.all([wait(100, 'marks'), wait(100, 'attendance')]);
  console.log(a, b, Date.now() - start < 180 ? 'ran in parallel' : 'ran one after another');
}
main();

Asha (CSE)
caught: server down
marks attendance ran in parallel

Rules worth remembering

• Wrap awaits in try/catch, or add .catch() to the promise. An unhandled rejection can crash a Node.js process.
• Two awaits in a row run one after the other. If the tasks do not depend on each other, start them together with Promise.all, as above.
• Promise.all rejects as soon as any task fails. Use Promise.allSettled when you want every result, failed or not.
• await only works inside an async function, or at the top level of an ES module.

How it fits with the event loop

Code after an await runs later as a microtask, after the current synchronous code finishes. See the Node.js event loop tip for the exact order. Practise more in UniCircle interview prep: JavaScript and TypeScript.