Function in JavaScript

Functions in JavaScript are a special kind of object. Function is an object that contains executable code. A function can be invoked to run that executable code and return a value.

Here is an example of a function.

function display() {
  console.log("Backbencher");
  console.log("JavaScript");
  console.log("Coding is fun");
}

In this function, display is the name of the function and the function contains 3 statements. The task to do for these statements is to print 3 strings to the console. So, when we call or invoke this function, it prints 3 texts to the console with the help of console.log method. display function is not calculating any value, instead it is performing a task.

We can also write functions to calculate a value and return the result. Here is a function sum() that calculates the sum of 2 numbers and returns the result.

function sum(a, b) {
  return a + b;
}

The function sum() can be used whenever we need to add 2 numbers. Therefore, functions help us in reusing code.

Defining Function

A function needs to be first defined to be called later. We can define a function using following ways in JavaScript.

Function declaration

Here we are defining a function using function keyword. The format is:

function <function name>(argument1, argument2, ...) {
  // function body
}

Here is an example.

function sum(num1, num2) {
  return num1 + num2;
}

Function Expression

When we assign a function to a variable using assignment operator(=), a function expression is formed.

var sum = function (num1, num2) {
  return num1 + num2;
};

Functions are True Values

Functions in JavaScript are true values. It means functions in JavaScript can be treated like regular objects.

function sum(a, b) {
  return a + b;
}
sum.myProperty = "Hello";
console.log(sum.myProperty); // "Hello"

Here we added a new property to sum() just like a regular object.