JS Promise (Part 1 - Basics)

What is Promises/A and Promises/A+ ?

  1. Both are specification for open standard but Javascript currently uses Promise/A+.
  2. The main three reason for using Promise/A+ is Three different states, Value for Fulfilment and for Rejection and thenable object. for more detail see Differences from Promises/A
  3. These Promise/A+ organisation occasionally revise and address the corner cases in to new Specification.

Difference between Promise/A and /A+?

  1. Rejection / Fulfilment method returns thenable object.
  2. Rejection triggers catch method, if errors thrown.
Promise API Flow (Image Credits to MDN — Mozilla Developer Network)
  1. It’s a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.
  2. It’s defined as an Object used for future result of an asynchronous computations.
  3. It represent a single asynchronous operation and gives a proxy until you get an actual value.
  4. Allows you to add handler to asynchronous of success or failure conditional actions.
  5. Return a promise instead of final value.
  6. It implements observer pattern (maintains a dependents and notifies them automatically of any state changes but can cause memory leaks, known as the lapsed listener problem) and its chain-able.
  7. It comes with integrated error handling and handles automatically fulfilled with value or reject with reason.
Observer Pattern — Image Credit to Wikipedia

Why Promises?

  1. Promises mainly to solve Callback Hell (heavy nested callback code) and Pyramid of doom Problem.
  2. Promises helps to group your asynchronous in a efficient way.
  3. Javascript is not a new candidate for adapting Promises/A+ Specification. its been already included by other famous programming languages like Java (sample), Scala (sample), Python (sample) and Clojure (sample).

Promise States

  1. pending: Initial Case where promise instantiated.
  2. fulfilled: Success Case which means promise resolved.
  3. rejected: Failure Case which means promise rejected.

Promise Methods

  1. Promise.all([promise1, promise2, …]);
  2. Promise.race([promise1, promise2, …]);
  3. Promise.reject(value);
  4. Promise.resolve(value);
  5. Promise.catch(onRejection);
  6. Promise.then(onFulFillment, onRejection);

Promise Iterable Methods

'use strict';// All method
// Trigger `catch` Method, if any one promise rejected.
// Trigger `then` Method, once every promise fulfilled.
Promise.all([promise1, promise2]);// Race method
// Trigger `catch` Method, if any one promise rejected.
// Trigger `then` Method, if any one promise fulfilled.
Promise.race([promise1, promise2]);

What is executor function ?

  1. Executor Functions are Parameter for Promise Constructor which holds Resolve and Reject Callbacks.
  2. It is executed immediately by the Promise implementation which provides the resolve and reject functions.
  3. It’s Triggered before the Promise constructor even returns the created object.
  4. The Resolve and Reject functions are bound to the promise to fulfill or reject.
  5. It’s expected to initiate some asynchronous work and then, once that completes, call either the resolve or reject.

What is the value of Promise.length?

Are You Tired with above Theories?

No Worries. Lets see quick and interesting questions with solutions from simple to advanced level !

How to create / instantiate the Promise?

'use strict';// ES5
var myPromise = new Promise(function (resolve, reject) {
// Your Code which you are unsure about execution time duration.
// Call resolve() method at the end of successful execution. // Call reject() method for your failure case. // Catch method will be called automatically, if any error occurs.
// ES6, Example with Arrow Functionsvar myPromise = new Promise((resolve, reject) => { ... })

How Promise methods can be overridden ?

How to do error handling in Promise ?

'use strict';// First ApproachyourPromise.catch(function (error) {
// Your Error Callback
// Second ApproachyourPromise.then(undefined, function (error) {
// Your Error Callback

How to detect whether Promise rejected ?

'use strict';Promise.catch(onRejected);
Promise.then(onFulfilled, onRejected);

How to combine multiple Promises in to one ?

'use strict';// UsagePromise.all([Promise1, Promise2, …]).then(onFulfilled, onRejected);// Examplevar promiseCall = function (waitSecond, returnData) {
return function (resolve, reject) {
setTimeout(resolve, waitSecond, returnData);
var p1 = new Promise(promiseCall(1000, "one"));
var p2 = new Promise(promiseCall(2000, "two"));
var p3 = new Promise(promiseCall(3000, "three"));
var p4 = new Promise(promiseCall(4000, "four"));
var p5 = new Promise(function (resolve, reject) {
reject('5th Promise Rejected');
// Calling Promise 1 - 4 in Promise.all()
Promise.all([p1, p2, p3, p4]).then(function (value) {
}, function (reason) {
// Not Called
// Expected Output: ["one", "two", "three", "four"]// Calling Promise 1 - 5 in Promise.all()
Promise.all([p1, p2, p3, p4, p5]).then(function (value) {
// Not Called
}, function (reason) {
// Trigger Rejection (Second) Callback if any one is rejected.
// Expected Output: 5th Promise Rejected

What is Promise Racing ?

'use strict';var p1 = new Promise(function(resolve, reject) { 
setTimeout(resolve, 500, "one");
var p2 = new Promise(function(resolve, reject) {
setTimeout(resolve, 100, "two");

Promise.race([p1, p2]).then(function(value) {
console.log(value); // "two"
// Both resolve, but p2 is faster

What Promise.all() return for invalid parameter ?

Promise.all() Example for Invalid Parameters

Why not Promise.race() with invalid parameter ?

Promise.race() Example for Invalid Parameters

What’s the main difference between callback-based functions and promise-based functions ?

  1. You don’t have to write error conditions `if (err) return callback(err)`.
  2. Callback need to be invoked the right callback immediately whereas Promise can be return as promise object and invoke later.
  3. In Promise, Anytime, When `.then()` method returned a promise and can be “unpacked” it to get to an actual value.
  4. In Callback, We must explicitly handle the error and we can’t deal with that error there.
  5. In Callback, We must call the passed callback to pass that error.

How many ways to resolve the Promise ?

var responseValue = ‘Success’;// First Approach
var directPromiseValue = Promise.resolve(responseValue);
// Second Approach
var instantiatePromise = new Promise(function (resolve) {

How to resolve with hard-coded value to Promise?

var directPromiseValue = Promise.resolve('Success');

How to reject with hard-coded value to Promise?

var directPromiseValue = Promise.reject('Custom Error Object');




Full Stack / Polyglot / React Developer ★ Redux ★ AWS ★ React Native ★ Flutter ★ Tech Writer ★ Open Source Enthusiast — webslate.io

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Full Stack / Polyglot / React Developer ★ Redux ★ AWS ★ React Native ★ Flutter ★ Tech Writer ★ Open Source Enthusiast — webslate.io

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