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.

• Benefits of Promises
  1. Improves Code Readability
  2. Better handling of asynchronous operations
  3. Better flow of control definition in asynchronous logic
  4. Better Error Handling  

• A Promise has four states:
  • fulfilled: Action related to the promise succeeded
  • rejected: Action related to the promise failed
  • pending: Promise is still pending i.e not fulfilled or rejected yet
  • settled: Promise has fulfilled or rejected

var promise = new Promise(function(resolve, reject){ //do something });

Parameters

• Promise constructor takes only one argument,a callback function.
• 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.

// javascript Promise
var myPromise=new Promise(function(resolve,reject){
  const x='hello';
  const y='hello';
  if(x===y){
    resolve();
  }
  else{
    reject();
  }
});
myPromise.then(function(){
  alert('Thank you this is promise resolve');
}).catch(function(){
  alert('Thank you this is promise reject');
});