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

Parameters:

• The promise constructor takes only one argument which is a callback function
• The callback function takes two arguments, resolve and reject
  • Perform operations inside the callback function and if everything went well then call resolve.
  • If desired operations do not go well then call reject.

A Promise has four states:

1. fulfilled: Action related to the promise succeeded
2. rejected: Action related to the promise failed
3. pending: Promise is still pending i.e. not fulfilled or rejected yet
4. settled: Promise has been fulfilled or rejected