Promise

What is promise ?
Promise is an object representing the eventual completion or failure of an asynchronous operation. A JavaScript promise object contains both the producing code and calls to the consuming code. It can be used to deal with asynchronous operation in JavaScript. Promise State: Pending: initial at state not get fulfilled or rejected Fulfilled /Resolved: Promise Completed Rejected: promise failed A pending promise can either be resolved with a value or rejected with a reason error.

Promise Steps

                
                  var promise = new Promise(function(resolve, reject){
                        reject(8);
                  });
                  promise.then(add)
                    .then(sub)
                    .then(multi)
                    .then(divide)
                    .then(function(msg){
                          console.log('Result : ' +msg);
                    }).catch(function(err){
                          console.log('Error : ' +err);
                  });

Promise All

What is promise all ?
The promise all method return a single promise from list ofpromisese when all promises fulfill. 
                
                  var promise1 = Promise.resolve();
                  var promise2 = Math.random() > 8 ? 0 : Promise.reject();
                  var promise3 = new Promise((resolve, reject) => {
                    setTimeout(() => resolve('saten'), 1000)
                  });

                  Promise.all([promise1, promise2, promise3])
                  .then((val) => {
                    console.log(val)
                  })
                  .catch((val) => {
                    console.log(val);
                  })