Introduction

Functions are one of the fundamental building blocks of javascript, it's a group of reusable code that can perform some task and executed when it is been called or invoked.

Declaration of Function

 function function1()
{
   return "John"
}

Calling a Function

function1()

function function1()
{
   return "John"
}

console.log(function1())

Output:

John

Types of Functions

• Named Function

In this function, the function is declared with by function keyword along with function name.

function function1()
{
    console.log("John1")
}

function1()

Output:

John1

• Hoisting Function

Function hoisting is a mechanism in which the JavaScript engine physically moves function declarations to the top of the code before executing them.

function1()

function function1()
{
    console.log("John1")
}

Output:

John1

• Anonymous Function

An anonymous function is a function without a name. An anonymous function is not accessible after its initial creation.

const function2 = function(){
   console.log("John2")
}

function2()

Output:

John2

• Immediately Invoked Function

The immediately invoked function is execute itself immediately after the declaration.

function1()

function function1()
{
   console.log("John1")
}
const function2 = function(){
   console.log("John2")
}
const function3 = 
  (function(){
   console.log("John3")
})()

function2()

Output:

John1
John3
John2

• Arrow Function

An arrow function is an anonymous function expression written with the fat arrow syntax ( => ).

Arrow functions are not hoisted.

const function4 = () => {
   console.log("John4")
}

function4()

Output:

John4

Scope of Function

Variables defined inside a function cannot be accessed from anywhere outside the function, because the variable is defined only in the scope of the function. However, a function can access all variables and functions defined inside the scope in which it is defined.

Accessing Its Calling Scope

let name ='John'
function function1()
{
   console.log(name)
}
function1()

Output:

John

Function Scope variable

function function1()
{
let name ='John'
   console.log(name)
}
function1()

console.log(name)

Output:

John
ReferenceError: name is not defined

Built-in Functions

eval()

eval() is used to convert string to executable arithmetic operations.

let sum = '4+3'
console.log(eval(sum))

Output:

7

isNaN()

The isNaN() function determines whether a value is NaN or not.

let string = 'John'
console.log(isNaN(string))

Output:

false

parseInt()

The parseInt() function parses a string argument and returns an integer.

let string = '6562'
console.log(parseInt(string))
console.log(typeof(parseInt(string)))

Output:

6562
number

typeof()

The typeof() function parses an argument and returns a data type of element.

let string = '6562'
console.log(typeof(parseInt(string)))

Output:

number