What is a promise?

Learn the promise states: pending, fulfilled and rejected.

What is a promise?

A promise is an object that represents the eventual result of an asynchronous operation. It is in one of three states: pending, fulfilled or rejected.

Once a promise settles, it becomes immutable: a fulfilled promise cannot go back to pending. This stability removes the confusion of callback hell and makes chaining easy. Promises are the modern replacement for callbacks.

Creating a promise

A promise is created with new Promise. The constructor takes a function that receives two parameters: resolve and reject. If the work succeeds, you call resolve with the value; if it fails, you call reject with the error.

const order = new Promise((resolve, reject) => {
  if (inStock) {
    resolve('shipped');
  } else {
    reject('out of stock');
  }
});

resolve and reject are called only once; later calls are ignored.

Exercise Write a stock-check promise

Write a function called checkStock. It takes a quantity parameter and returns a Promise. If quantity is greater than 0, call resolve with true; otherwise call reject with the string 'out of stock'.

Show example solution

function checkStock(quantity) {
  return new Promise((resolve, reject) => {
    if (quantity > 0) {
      resolve(true);
    } else {
      reject('out of stock');
    }
  });
}

Test yourself

What happens after a promise becomes fulfilled?
- It can return to pending
- Its state stays fixed and its value does not change
- It automatically becomes rejected
- It must create a new promise
Show answer

Its state stays fixed and its value does not change — Correct! Once a promise settles, its state and value stay fixed.

Fulfil the promise

const loaded = new Promise((resolve, reject) => {
  ____;
});

Show answer

const loaded = new Promise((resolve, reject) => {
  resolve(data);
});

Match states to their meanings

pending

Result still awaited
fulfilled

Success, value ready
rejected

Failure, error present

Next lesson: Catching success with then

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