Using promises for asynchronous execution.

Promises.js

/*
Suppose I want to add two numbers, then double it, then square that number, then add double it again. Using Promises. 
 */
addTwoNos(2, 3)
    .then(sum => double(sum))
    .then(doubleOfSum => square(doubleOfSum))
    .then(square => double(square))
    .then(output => console.log(output))
    .catch(err => console.log(err));

function addTwoNos(num1, num2) {

    return new Promise((resolve, reject) => {
        if(isNaN(num1) || isNaN(num2)){
            reject(new Error('argument is not a number') )
        }
        else {
            resolve(num1 + num2)
        }
    })
}


function double(num) {

    return new Promise((resolve, reject) => {
        if(isNaN(num)){
            reject(new Error('argument is not a number') )
        }
        else {
            resolve(num * 2)
        }
    })

}


function square(num) {


    return new Promise((resolve, reject) => {
        if(isNaN(num)){
            reject(new Error('argument is not a number') )
        }
        else {
            resolve(num * num)
        }
    })

}