What are promises?

Learn the Promise concept and its states.

What is a Promise?

In JavaScript, a Promise is an object representing the eventual result of a long-running operation. A Promise is in one of three states: pending, fulfilled or rejected. Once it moves from pending to fulfilled or rejected, the state never changes again.

A Promise is created with new Promise((resolve, reject) => {...}), which takes an executor function. The executor calls resolve(value) on success or reject(error) on failure. This lets you wait for an asynchronous result without blocking the program; you chain code that runs when the result arrives.

The Promise constructor

To create a Promise you write new Promise(executor). The executor takes two parameters: resolve and reject. When the async work is done you call resolve(value) to deliver the result; on error you call reject(error).

const p = new Promise((resolve, reject) => {
  setTimeout(() => resolve("ok"), 100);
});

Here setTimeout calls resolve after 100 ms, so p moves to fulfilled. The executor runs synchronously; the Promise stays pending until resolve or reject is called.

Exercise Your turn: create a Promise

Create a Promise with new Promise((resolve, reject) => {...}). Inside the executor, use setTimeout to log a message after 10 ms and then call resolve("ready").

Show example solution

new Promise((resolve, reject) => {
  setTimeout(() => {
    console.log("working");
    resolve("ready");
  }, 10);
});

Test yourself

What happens after a Promise moves from pending to fulfilled?
- It can return to pending
- It can move to rejected
- A new Promise is created
- It never changes again
Show answer

It never changes again — Once a Promise is fulfilled, its state is locked.

Fill in the blank: deliver the result with resolve

const p = new Promise((resolve, reject) => {
  setTimeout(() => ____("ok"), 100);
});

Show answer

const p = new Promise((resolve, reject) => {
  setTimeout(() => resolve("ok"), 100);
});

Match the Promise states

pending

waiting
fulfilled

completed
rejected

failed

Next lesson: then and catch

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