Javascript – How to Define & Process a Promise Object

This article represents tips and code samples on How to define and process a Promise object. Please feel free to comment/suggest if I missed to mention one or more important points. Also, sorry for the typos.

Following are the key points described later in this article:

  • How to Define a Promise Object
  • How to Process a Promise object
How to Define a Promise Object

Following demonstrates the Auth module with an API, login, that returns a Promise that will be resolved/fulfilled or rejected later with appropriate object representing domain object such as User or error object with status and error message as demostrated below.

// Promise needs to be imported when executing a module using Node
var Promise = require("promise");

function Auth() {

Auth.prototype.login = function(user) {
    return new Promise(function(resolve, reject) {
        if(user.username != "" && user.password != "") {
            resolve({username: user.username, age: 21, location: "Hyderabad", firstname: "Raju", lastname: "Mastana"});
        } else {
            if(user.username == ""){
                reject({status:1, message: "Username is invalid"});
            } else {
                reject({status:2, message: "Password is invalid"});

module.exports = Auth;
How to Process a Promise object

Pay attention to some of the following in the code sample below which calls login API on an Auth object:

  • Create an Auth object
  • Invoke login API on Auth object
  • Invoke “then” method on Promise object returned from invocation of login API.
// Import the Auth module
var Auth = require('./Auth');

// Create an Auth object
var auth = new Auth();
// Calls login API on Auth object
// Returns a promise object
var loginPromise = auth.login({username:'', password:'pass123'});
// Once the state of Promise is fulfilled/resolved or rejected,
// following is executed
    console.log("User name: " +;
}, function(error) {


Ajitesh Kumar
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