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.then()

Registers callbacks for the fulfillment (and optionally rejection) of a promise and returns a new promise chained to the callback's return value. Returning a value fulfills the next promise; throwing or returning a rejected promise rejects it.

promise.then(onFulfilled, onRejected?)

Parameters

Parameter Purpose
onFulfilled(value) Called with the resolved value
onRejected(reason) Optional; catches rejections in this chain link
returns A NEW promise — enables chaining

Examples

fetch('/api/user')
  .then(r => r.json())
  .then(user => console.log(user.name));

Each .then unwraps the returned promise for the next handler.

Promise.resolve(1)
  .then(n => n + 1)
  .then(n => { throw new Error('nope'); })
  .then(n => console.log('skipped'), err => console.error(err.message));

A thrown error skips subsequent fulfillment handlers until an onRejected is found.

// Prefer .catch over the second argument
fetch(url).then(r => r.json()).catch(err => console.error(err));

Two-argument form doesn't catch errors thrown INSIDE onFulfilled — .catch does.

Gotcha

The onRejected argument of .then does NOT catch errors thrown in its own onFulfilled — chain a .catch instead. Forgetting to return the inner promise breaks the chain (the outer .then resolves before the inner work finishes).

Related

.catch()
Attaches a rejection handler to a promise chain, equivalent to .then(undefined, onRejected). Returns a new promise that fulfills with the handler's return value, letting you recover from errors.
.finally()
Runs a callback after the promise settles regardless of outcome, without changing the resolved value or rejection reason. Ideal for cleanup like hiding spinners or closing connections.
await
Pauses execution of the surrounding async function until the awaited promise settles, then resumes with its resolved value. If the promise rejects, await throws the reason, letting you use try/catch.

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