Free Computer Courses

How to use promises

Updated September 2026 · Beginner-friendly guide

Understand promise states

A promise starts pending and later becomes fulfilled with a value or rejected with a reason. Settlement happens only once.

const delay = milliseconds => new Promise(resolve => {
  setTimeout(resolve, milliseconds);
});

delay(200).then(() => console.log("Finished"));

Chain asynchronous work

Return values and promises from then callbacks so the next step receives them. End with catch for rejection.

Promise.resolve(5)
  .then(number => number * 2)
  .then(result => console.log(result))
  .catch(error => console.error(error));

Use async and await

An async function always returns a promise. await pauses that function—not the whole program—until settlement.

async function run() {
  await delay(100);
  return "Ready";
}

run().then(console.log);

Handle failures with try/catch

Wrap awaited operations that may reject. Preserve useful error context and decide whether to recover or rethrow.

async function loadValue() {
  try {
    const value = await Promise.reject(new Error("Unavailable"));
    return value;
  } catch (error) {
    console.error("Could not load:", error.message);
    return null;
  }
}
loadValue();

Related tutorials