High Order Functions in JavaScript

High Order Functions in JavaScript

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As a JavaScript developer you will utilize high order functions frequently, so having a decent comprehension of these functions is vital. Presently I see individuals get frequently confounded when finding out about the reduce() technique, yet I had clarified everything in detail so attempt to comprehend it bit by bit and I'm certain you will dominate it.

What are high order functions?

In a nutshell, high order functions are those functions that take other functions as arguments OR returning other functions. The function passed as arguments in high order function is known as callbacks.

Why use High Order Function?

  • They help us to write clean and simple code
  • Since the code will be clean, it will be easier to debug

Now javascript have some built-in high order functions, you may have already be using them without even realising. (filter(), reduce(), sort(), forEach()).


The filter method returns a new array of elements that passes a specific test provided by a callback function. And since filter takes a callback function, therefore filter() is known as high order function. Now the callback function that is passed into filter() is known as the high order function.

  • value of the element (required)
  • index of the element (optional)
  • the array object (optional)
let arr [1,2,3,4,5]; 

const resultant Array = arr.filter((element ) => {
    return element > 3; 

console.log(resultantArray); // [4, 5]

In the above example, what's happening is that the elements of the arr array are getting passed one by one into the filter() callback method and they are getting tested for a specific test that is element > 3 and those elements which are passing the test are getting pushed in the resultantArray, that's why the output is [4,5] since 4 and 5 were the only elements that pass the test.

The element argument is getting the value of elements of arr array one by one, it will first become 1 and then it will test 1>3 if it's true 1 will get pushed in the resultant array and if it's false it will be skipped to the next element.


// filter age that is less than 18

const ageArray = [10, 12, 35, 55, 40, 32, 15]; 

const filterAgeArray = ageArray.filter((age)=> {
    return age < 18; >

// [10, 12, 15]


// /filter positive numbers

const numArray = [-2, 1, 50, 20, -47, -40]; 

const positiveArray = numArray.filter((num) => {
    return num > 0; 

// [1, 50, 20]


// filter names that contains `sh` in it

const namesArray = ["samuel", "rahul", "harsh", "hitesh"]; 

const filterNameArray = namesArray.filter((name) =>{
    return name.includes("sh"); 

// ["harsh", "hitesh"]


As the name suggests, the map() method is used to map the values of an existing array to new values and it pushes that new values to a new array and it returns that new array. Now map() also takes a callback function and hence it is known as high order function.

Now the callback fucntion that is passed into map() method takes three arguments:-

  • values of the element (required)
  • index of the element (optional)
  • the array object (optional)
const numArray = [1, 5, 3, 6, 4, 7]; 

const increasedArray = numArray.map((element) => {
    return element + 1; 

[2, 6, 4, 7, 5, 8]

Just like in filter(), the elements of numArray will be passed one by one into the map() callback function (as the element argument) and they will get mapped into a new value that is element + 1 and then they will be pushed into the increasedArray.

Firstly 1 will get a pass as an element argument and it will get the map to a new value that is element + 1 such that 1 + 1 (because here element is 1) and the 2 will get pushed into increased Array and so on for 5, 3, 6, 4, 7.


// exponentiate every number on an array

const numArray = [2, 3, 4, 5, 15]; 

const poweredArray = numArray.map((number) => {
    return number * number; 

// [4, 9 ,16, 25, 144, 225]
// extract the marks of student

const studentsArray = [
        name: "Rahul", 
        marks: 45, 
        name: "Samuel", 
        marks: 85, 
        name: "Chris", 
        marks: 25, 

const ScoreArray = studentsArray.map((student) => {
    return student.marks; 

// [45, 85, 25]


The reduce() method is used to reduce the array to a single value, just like filter() and map(), reduce() also takes a callback function as an argument hence it is known as High Order Function. BUT. reduce() takes one more argument other than the callback function and that is initialValue. And again like filter() and map() the callback function passed into reduce() takes some arguments but the callback functions passed into reduce takes 4 arguments instead of 3.

  • total (required)
  • value of the elements (required)
  • index of the element (optional)
  • the array object (optional)
// A basic example reduce()

const numArray = [1, 2, 3, 4, 5]; 

const sum = numArray.reduce((total, num) => {
    return total + num; 


Let's first understand what total argument is:- Total argument is the previous value returned by reduce() function, now when the reduce() will run for the first time there will be no previous returned value therefore for the first time total argument is equal to the initialValue(remember the second argument that we passed into reduce()).

Now we also haven't used the initialValue in our example, so what is that when we don't pass initialValue, the reduce() method skips the first element of the numArray becomes the value of total argument.

Coming to our example, we haven't passed initialValue so the first element of numArray such that 1 will become the value of total argument and the second element of numArray will pass as num argument, and the reduction will return total + num such that 1+2 = 3, 3 will become the new value of total and now the third element from numArray will get a pass into reduce() callback as num argument, again reduce will return total + num that is 3 + 3 = 6 and 6 will become the new value of total and so on.

(This explanation is a bit tough and confusing. If you try to learn step by step you will master reduce()).

The initialValue argument

initialValue as the name suggests, is the initial value of the total argument, as we know when reduce() runs for the first time there is no previous returned value and hence the first element from existing array (numArray in our case) becomes the value of the total argument, so instead of doing that we can give an initial value to the total argument. (remember initialValue will be the initial value of the total argument, the total argument will become the previous return value of reduce() later).

Note:- When you will use the initialValue argument, numArray will not skip it's the first element hence every element will get passed into the reduce() callback.

Syntax of reduce() with initial value:-

const resultantArray = existingArray.reduce((total,element,index.array)=> {
    // return something
}, initialValue);

Thank you Reading⚡

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