Higher order function JavaScript

 A function takes another function as an argument and return a function as a value. it is known as Higher Order Function.

JavaScript functions are first class citizen

function sayHello() {
    return function(){
          return "Hello world";
        }
}
let fn = sayHello();
let message = fn();

Higher order function

let number = [1,2,3];
number.map(number =>number*2)

map() is a higher order function in JavaScript becouse it take a function as an argument. another example of higher order function is setTimeOut()

setTimeOut(()=> console.log("Hello"), 1000);

Ex. (normal method )

const radius = [3, 1, 2, 4];
// calculate the area of this four circles
const calculateArea = function(radius){
    const temp = []
    for(let i = 0; i<radius.length ; i++ ){
        temp.push(2*Math.PI*radius[i]*radius[i])
    }  
    return temp  
}
console.log(calculateArea(radius))
// calculate cercumference of this four circle 
const carcumference = function(radius){
    const temp = []
    for(let i = 0; i<radius.length; i++){
        temp.push(2*Math.PI*radius[i])
    }
    return temp
}
console.log(carcumference(radius))

[
  56.548667764616276,
  6.283185307179586,
  25.132741228718345,
  100.53096491487338
]
[
  18.84955592153876,
  6.283185307179586,
  12.566370614359172,
  25.132741228718345
]

Using higher order function ( Don't Repeat Yourself) 

const radius = [3, 1, 2, 4];
const area = function (radius) {
    return Math.PI * radius * radius
}
const circumference = function (radius){
    return 2*Math.PI *radius 
}
const calculate = function (radius, logic) {
    const temp = []
    for (let i = 0; i < radius.length; i++) {
        temp.push(logic(radius[i]));
    }
    return temp
}
console.log(calculate(radius, area))
console.log(calculate(radius, circumference))

[
  28.274333882308138,
  3.141592653589793,
  12.566370614359172,
  50.26548245743669
]
[
  18.84955592153876,
  6.283185307179586,
  12.566370614359172,
  25.132741228718345
]