A promise is an object that may produce a single value some time in the future: either a resolved value, or a reason that it’s not resolved (e.g., a network error occurred). A promise may be in one of 3 possible states: fulfilled, rejected, or pending. Promise users can attach callbacks to handle the fulfilled value or the reason for rejection.

Promises are eager, meaning that a promise will start doing whatever task you give it as soon as the promise constructor is invoked.

Promises states:

  • Fulfilled: onFulfilled() will be called (e.g., resolve() was called)
  • Rejected: onRejected() will be called (e.g., reject() was called)
  • Pending: not yet fulfilled or rejected
const wait = time => new Promise((resolve) => setTimeout(resolve, time));
wait(3000).then(() => console.log('Hello!')); // 'Hello!'



Another explanation

Promises are used to handle asynchronous operations in JavaScript. They are easy to manage when dealing with multiple asynchronous operations where callbacks can create callback hell leading to unmanageable code.

Prior to promises events and callback functions were used but they had limited functionalities and created unmanageable code.
Multiple callback functions would create callback hell that leads to unmanageable code.
Events were not good at handling asynchronous operations.

promise. then(function () { console.log('Success, You are a GEEK'); }). catch(function () { console.log('Some error has occured'); });

Promises are the ideal choice for handling asynchronous operations in the simplest manner. They can handle multiple asynchronous operations easily and provide better error handling than callbacks and events.


Success, You are a GEEK