Programming Fundamentals › Concurrency & Async

Promise / Future

An object representing a value that will be available later.

Also known as: Promises, Future, promise object, Promise API

A promise (called a Future in some languages) is an object standing in for a value that isn’t ready yet, such as the result of a network request.

A promise is always in one of three states:

Once it leaves pending, it never changes.

fetch("/api/user")                    // returns a promise immediately
  .then((res) => res.json())          // runs when it's fulfilled
  .then((user) => console.log(user.name))
  .catch((err) => console.error(err)) // runs if any step rejects

Each .then() returns a new promise, so you can chain steps in order instead of nesting callbacks inside callbacks (see callback hell). Modern code usually writes this with async/await, which is built on promises.

Mistakes to avoid

• Forgetting that it’s a promise. const user = fetch(...) gives you a promise, not the user. You need .then or await.
• Not handling rejections. A rejected promise nobody catches shows up as an “unhandled rejection” and can crash a Node process or fail silently in a browser. Always .catch or use try/catch with await.
• Running things one by one that could run together. Use Promise.all for independent requests.
• Forgetting to return inside .then, which passes undefined to the next step.

Other languages have the same idea under other names: Python’s asyncio.Future, Java’s CompletableFuture.