Promises

What is a Promise?

A Promise is an object representing a value that will be available in the future. It has three states:

Pending — the operation is still in progress.
Fulfilled — the operation completed successfully; the Promise holds a value.
Rejected — the operation failed; the Promise holds an error.

A Promise transitions from pending to either fulfilled or rejected — never back. Once settled, it stays that way.

var p = new Promise(function(resolve, reject) {
  // Call resolve(value) to fulfill, or reject(error) to reject
  resolve('hello');
});

Chaining with .then / .catch / .finally

.then(onFulfilled) returns a new Promise, allowing you to chain steps. .catch(onRejected) handles rejections. .finally(fn) runs regardless of outcome — useful for cleanup.

fetchData()
  .then(function(data) { return process(data); })
  .then(function(result) { console.log('Done:', result); })
  .catch(function(err) { console.error('Failed:', err.message); })
  .finally(function() { console.log('Cleanup'); });

Promise combinators

Combinator	Behavior
`Promise.all(arr)`	Resolves when all resolve; rejects on the first rejection
`Promise.race(arr)`	Settles with the first Promise to settle (fulfilled or rejected)
`Promise.allSettled(arr)`	Resolves when all settle; gives each outcome (never rejects)
`Promise.any(arr)`	Resolves with the first fulfilled; rejects only if all reject

Runnable demo

The snippet creates two Promises that resolve immediately with fixed values, chains .then, uses Promise.all, and adds .finally for cleanup.

JavaScript

var p1 = Promise.resolve('data-A');
var p2 = Promise.resolve('data-B');

p1.then(function(data) {
  console.log('Fetched: ' + data);
  return Promise.all([p1, p2]);
}).then(function(results) {
  console.log('All done: ' + results[0] + ' and ' + results[1]);
}).catch(function(err) {
  console.log('Error: ' + err.message);
}).finally(function() {
  console.log('Finally: cleanup');
});

The output is always: Fetched: data-A, then All done: data-A and data-B, then Finally: cleanup.