All 15 Async Patterns questions
More in Async Patterns
What is a callback function in Node.js, and what is 'callback hell'? How do you convert a Node.js callback-based function into a Promise-based one manually, without using util.promisify()? What is the purpose of the 'node:timers/promises' module, and how does it simplify working with delays in async code? What is the difference between Promise.all(), Promise.allSettled(), Promise.race(), and Promise.any()? How does async/await relate to Promises under the hood, and what does 'await' actually do to program execution? What is the difference between running async operations sequentially with multiple awaits versus concurrently with Promise.all()? How do you implement a retry-with-exponential-backoff pattern for a flaky async operation in Node.js? What is util.promisify(), and when would you still need it in modern Node.js? What is the difference between micro tasks and macro tasks in Node.js's event loop, and how does this affect Promise callback ordering? What is an async generator, and how does 'for await...of' let you consume asynchronously produced data?
Advertisement
Back to results for “Error Handling”
Node.js › Async Patterns

What is a Promise in JavaScript, and what are its three possible states?

Beginner

Answer

A Promise is an object representing the eventual result of an asynchronous operation, existing in exactly one of three states at any time: pending (the operation hasn't completed yet), fulfilled (the operation succeeded, with a resulting value), or rejected (the operation failed, with a reason/error) -- once a promise transitions to fulfilled or rejected it is permanently 'settled' and cannot change state again, which is what makes chaining .then()/.catch() predictable.
const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    const success = Math.random() > 0.5;
    success ? resolve('Data loaded') : reject(new Error('Load failed'));
  }, 1000);
});

promise.then(result => console.log(result)).catch(err => console.error(err.message));
Real-world example A data-fetching function returns a Promise that resolves with the parsed API response or rejects with a descriptive error, letting every caller consistently use .then()/.catch() or await/try-catch rather than needing to know the internal implementation details of how the request was made.

Common follow-ups: What happens if resolve() is called after reject() has already been called on the same promise?;How does Promise.prototype.finally() differ from adding logic inside both .then() and .catch()?

Async Patterns;Error Handling
All 15 Async Patterns questions