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Promises In JavaScript are basically used to handle operations asynchronous operations. A promise is an object which may produce a single value in the future: either a resolved value, or an error.
Following pointers will be covered in this article,
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The promises come in handy when there are multiple asynchronous operations to be dealt with. Before promises were introduced in JavaScript, there were events and callback functions being used to handle asynchronous operations. Since events are not much useful in case of asynchronous operations, they are not preferred. Coming to callback, using them in multiples would create such a mess that it was very difficult for anyone to understand the code.
Therefore Promises are the first choice of every coder to handle asynchronous operations in the simplest manner. They have advanced features which makes it easy for them to handle the operations than callback and events.
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Fulfilled: Related to those promises which are succeeded.
Rejected: Related to those promises which are rejected.
Pending: Related to those promises which are pending i.e. neither rejected nor accepted.
Settled: Related to those promises which are being fulfilled or rejected.
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A promise is created using promise constructor.
Syntax:
var promise = new Promise(function(resolve, reject){ //do something here });
Parameters:
Promise constructor takes one argument, callback function. There are two arguments in callback function, resolve or reject. Operations are performed inside the callback functions, if everything went well then call resolves otherwise call gets rejects.
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Example 1:
var promise = new Promise(function(resolve, reject) { /*declaring and defining two variables of const data type with same content.*/ const a = "Hello there! My name is Yash and I am interested in computer Science."; const b = " Hello there! My name is Yash and I am interested in computer Science."; //checking if both the content stored in variables are same or not if(a === b) { //calling resolve resolve(); } else { //calling reject reject(); } }); promise. then(function () { console.log('Promise Resolved!!'); }). catch(function () { console.log('Promise Rejected!!'); });
Output:
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Example 2:
var promise = new Promise(function(resolve, reject) { //initializing two variables with integer values const x = 11+2; const y = 26/2; //checking if both variables are equal or not if(x === y) { //calling resolve resolve(); } else { //calling reject reject(); } }); promise. then(function () { console.log('Promise is Resolved!!'); }). catch(function () { console.log('Promise is Rejected!!'); });
Output:
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Example 3:
var promise = new Promise(function(resolve, reject) { const i = "Hello"; const a = "World"; //performing addition of two variables to store value in another variable const j= i+a; if((i+a) === j) { //calling resolve resolve(); } else { //calling reject reject(); } }); promise. then(function () { console.log('Promise is Resolved!!'); }). catch(function () { console.log('Promise is Rejected!!'); });
Output:
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There are two registering functions:
then()
When a promise is either resolved or rejected, then() is invoked.
Parameters:
Syntax:
.then(function(result){ //handling success }, function(error){ //handling the error })
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Example
//resolving of promise var promise = new Promise(function(resolve, reject) { resolve('Success message is written here!'); }) promise .then(function(successMessageishere) { //success handling function is invoked console.log(successMessageishere); }, function(errorMessageishere) { console.log(errorMessageishere); })
Output:
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//Rejection of promise var promise = new Promise(function(resolve, reject) { reject('Rejection message is written here!') }) promise .then(function(successMessage) { console.log(successMessage); }, function(errorMessage) { //error handler function is invoked console.log(errorMessage); })
Output:
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Catch()
Whenever there is some sort of error or the promise gets rejected during execution time, catch() is invoked.
Parameters:
Syntax:
.catch(function(error){ //handling error })
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Example 1:
var promise = new Promise(function(resolve, reject) { reject('Promise is Rejected') }) promise .then(function(success) { console.log(success); }) .catch(function(error) { //error handler function is invoked console.log(error); });
Output:
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Example 2:
var promise = new Promise(function(resolve, reject) { //error message throw new Error('There is some error!') }) promise .then(function(success) { console.log(success); }) .catch(function(error) { //error handler function is invoked console.log(error); });
Output:
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Example 3:
var promise = new Promise(function(resolve, reject) { //error message can be edited here throw new Error('some error occured !') }) promise .then(function(Thissuccess) { console.log(Thissuccess); }) .catch(function(Thiserror) { //error handler function invoked console.log(Thiserror); });
Output:
Application:
1. Handling asynchronous events.
2. Handling asynchronous http requests.
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