What is Functions in JavaScript

Hey, guys, welcome back, in this tutorial, we will learn what is functions in JavaScript. So, let’s start.

What is functions in JavaScript?

Like other programming languages, a function in JavaScript is a block of code that is designed to perform a particular task. It is executed when it is invoked or called.

what are functions in javascript

A JavaScript function is defined using the function keyword followed by the function name and parentheses (). A sequence of statements must be enclosed inside curly braces {}.

In other words, a function is a set of statements known as the function body that takes some inputs and performs some operations and produces the output.

In JavaScript, a function consists of three parts:

  • Function declaration
  • Function body or definition
  • Function call

Function Declaration in JavaScript

A JavaScript function is declared using a function keyword followed by a function name and parentheses. The function name follows rules similar to naming a variable.

function function_name (param1,param2,...){
  // statements

Here, param1,param2… are parameters passed to the function and are optional.

Function body or function definition in JavaScript

A set of statements enclosed inside curly braces {} is called a function body or function definition.

Function Calling in JavaScript

A function is called or invoked using the function name followed by parentheses (). An argument is also passed if any.

function fun() // function declaration
  // function body or definition
  console.log(“Hello, World!”);
fun();   // function call

A function may or not return a value depending upon the requirement. In the above example, the function fun() does not return anything.

function sum(num1,num2){
  return num1+num2; // returning value using return keyword
console.log(sum(4,5)); // function call

Flow of execution of function in JavaScript

When a function is called or invoked, the program control reaches the function body. If the function is returning any value then it is printed or stored in a variable.

flow of execution of function

Why use functions in JavaScript?


We can define a function once and can be used later whenever required just by calling it.

For example, suppose you want to send a greeting message to 10 friends along with their names, say “Good Morning, Rahul”. So, you have to write it 10 times. The solution is to write the message once and change the name according to your friend’s name you want to send.

function greeting_msg(name){
  console.log(“Good Morning ”, name);


There are many predefined functions in JavaScript, so the programmer has to just call it without knowing its working or implementation details.

let a=8;
console.log(a); // 8
console.log(typeof a); // number
let b = String(a); // converting number to string
console.log(b); // 8
console.log(typeof b); // string

Here, we are just calling the String() function to convert a number into a string.

Reduces program complexity

The whole program is divided into small modules or parts that make the program easy to understand and increase the program’s readability.


Debugging of code becomes easier and errors can be easily traced out.

Types of Functions in JavaScript

  1. Pre-defined function
  2. User-defined function

Pre-defined function in JavaScript

These are built-in functions in Javascript. You have to just call it.
For example, Number(), search() etc.

let a="28.4";
console.log(a); // 28.4
console.log(typeof a); // string
let b=Number(a); // converting string to number
console.log(b); // 28.4
console.log(typeof b); // number
const str = "My name is Rohit";
const str1=str.toLowerCase();
console.log(str1); // my name is rohit.

User-defined functions in JavaScript

These functions are defined by the user.

function add(){
  console.log(4+7); // 11

Parameters and Arguments in JavaScript

A function parameter is a variable that is defined during the function declaration. It is also called a formal parameter. These are used to hold the variables or values that are passed during the function call. A function argument is a value or variable passed during the function call. It is also called the actual parameter.

function multiply(num1,num2){

let a=4,b=5;

In the above example, num1 and num2 are called formal parameters whereas a & b are called actual parameters or arguments.

Default Parameter in JavaScript

A default parameter is a parameter that takes a default value when arguments or actual parameters are not passed.

function fun(name,age=21){
  console.log("Name",name); // Rohit
  console.log("Age", age); // 22

fun("Rohit",22); // function call 1
fun("Muskan"); // function call 2

In the above example, 22 is printed in the case of function call 1 because 21 is passed during the function call but in the case of function call 2, 21 is printed because no value is passed during the function call.

Function Expression in JavaScript

In JavaScript, we can also use a function keyword to define an expression. The main difference between function and function expression is that the function expression may or may not have a function name. It is used to create an anonymous function i.e., a function without a name. The function is stored in a variable & called or invoked using a variable name followed by parentheses.

A function expression can be used as an IIFE (Immediately Invoked Function Expression) which means that it will be executed as soon as it is defined.


let variable_name=function(paramters){
  // code

const variable_name=function function_name(parameters){
  // code

variable_name(arguments); // function expression call


// anonymous function
let area = function(radius){
  console.log(3.14*radius*radius); // 13.8474


const perimeter=function rectangle(l,b){
  return 2*(l+b);


I hope you have understood what is functions in JavaScript and you might be excited to implement them. For more tutorials, like these, stay connected with us, until then stay safe, stay healthy and keep learning!

Thanks for reading.