Javascript Introduction
Comments in Javascript
Javascript Variables
Difference between var, let and const in Javascript
Datatypes
Assignment Operators
Logical Operators
Comparison Operators
Actions alert, prompt & confirm

Javascript Flow Control
Conditional Statements in Javascript
Nullish coalescing operator in Javascript
Break and Continue in Javascript
Loops in javascript
for loop in Javascript
while loop

Javascript Functions
Javascript Functions
JavaScript Arrow Function

Javascript Objects
Javascript Objects

Javascript Array
Javascript Arrays

Javascript Error Handling
Error Handling in Javascript

Javascript ES6
Set
Javascript Spread Operator
Rest Parameter in Javascript

Javascript Asynchronous
Javascript Callback Function
Promises in Javascript
Promise Chaining
Async-Await in Javascript

Javascript DOM
DOM Document
DOM Elements
DOM Attributes
DOM Manipulation
DOM Node Lists
Javascript Events
DOM Traversing Elements
DOM EventListener

Javascript Class
Classes in javascript
Class Inheritance

Javascript Design Principles
SOLID - The Single responsibility principle in javascript
SOLID - Open-Closed principle in Javascript

Other Topics
Javascript Hoisting
Filter, Map and Reduce functions in Javascript
Closure in Javascript
Optional chaining operator in Javascript
Consoling in table format in javascript
Fetch API in Javascript
HTML web Storage API
Checking the online or offline status of the browser

Promises in Javascript

Promises are used in javascript to perform any asynchronous operation. Its one of the solution for the callback hell. When multiple callback would create callback hell which will lead to unmanaged code.

A promise is something like promising that the task would be completed in near future either successfully or return an error.

The syntax of the promise looks like as below.

        let promise = new Promise(function(resolve, reject){
        //do something.
    });

  

Here the promise constructor takes a single argument which is a callback function. The callback function takes two arguments. First argument is resolve and the second argument is reject. If the task is completed successfully then we pass the result in resolve function. If the task fails then call the reject function.

Let's see an example for the promise

    const number = 6;
function checkEven(number) {
    return new Promise((resolve,reject)=> {
        if(number % 2 === 0) {
            resolve(`${number} is an even number.`);
        } else {
            reject('Not an even number');
        }
    });
}

checkEven(number)
  .then(data => {
    console.log(data);
  })
  .catch(error => {
    console.log(error);
  });
  console.log('End of the program.');

  

Output:

End of the program.
6 is an even number.

In the above program we have a function named checkEven which gets a number as an argument. This function would return a promise as response. If the number provided is an even number then resolve function is called with a message that '6 is an even number'. If the number is an odd number, say suppose its number 7 then it would call the reject function with an error message "Not an even number". In the above code when resolve is called, the callback function in the then block gets called which get the parameter returned by the resolve function. If the reject function is called then the catch block will be executed with the parameter returned by reject function.

If we note the above output, the line "End of the program" get printed first even though its present at the last line of the program. This clearly denotes that promise is an asynchronous function and javascript doesn't wait for it to execute the next line.

Promises in Javascript

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