What is a promise in Javascript?

A Promise is an object returned by an asynchronous method.

A Promise is a proxy for a value not necessarily known when the promise is created. It allows you to associate handlers with an asynchronous action's eventual success value or failure reason. This lets asynchronous methods return values like synchronous methods: instead of immediately returning the final value, the asynchronous method returns a promise to supply the value at some point in the future.

Why should I use promises?

The promises will help you to make all the complex async code easier to handle, understand and maintain.

Promises are not about callback aggregation. That’s a simple utility. Promises are about something much deeper, namely providing a direct correspondence between synchronous functions and asynchronous functions.

The following are the states of promise

• pending: initial state, neither fulfilled nor rejected.
• fulfilled: meaning that the operation was completed successfully.
• rejected: meaning that the operation failed.

States of Promise

Chained Promises

The methods promise.then(), promise.catch(), and promise.finally() are used to associate further action with a promise that becomes settled.

The .then() method takes up to two arguments; the first argument is a callback function for the resolved case of the promise, and the second argument is a callback function for the rejected case. Each .then() returns a newly generated promise object, which can optionally be used for chaining; for example:

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('foo');
  }, 300);
});

myPromise
  .then(handleResolvedA, handleRejectedA)
  .then(handleResolvedB, handleRejectedB)
  .then(handleResolvedC, handleRejectedC);