Promises-in-js

Promises in the JavaScript

Promises in JavaScript are used to use the javascript Single Threaded very wisely. in which a promise takes a argument as a function (a call back) with 2 parameters namely

1. Resolve
2. Reject

• Resolve in sense that the callback is a sucess.
• Reject in sense that the callback is a failure.

so Defining a simple promise loks like this

let mypromise=new Promise(function(resolve,reject){
        //resolve();
        //or
        //reject();
});

Using a promise in the program

promises are mainly used in the program when we want a task to be started only the other task is completed usually events are used to do this kind on stuff in js, but it becoms messy for large number of events so promises may be used to improve the readability of the program and reduces nightmares

using a promise in a js program is as follows

//defining a promise
let mypromise=new Promise(function(resolve,reject){
			var d = new Date();
			console.log("called a function at the time of "+d.getSeconds());
			setTimeout(function(){return resolve();}, 2000);
 	    	  });

//using the promise
mypromise.then(function(){
 			var d = new Date();
			console.log("promise resolved at "+d.getSeconds());
	  })
	 .catch(function(){
	  	var d = new Date();
	  	console.log("promise failed "+d.getSeconds())
	});

#output:-

 called a function at the time of 54
 promise resolved at 56

in the above program when the promise is resolved then the then callback is executed and if it is rejected catch callback is executed as the promise alwase resolve the then call back is executed.

Where promises are used

• Promises are mostly used in place of events when two functions are dependent on each other.
• Service Worker Api mainly uses the promises .

Best video explanation

Promise Explained The Best Video