Functions

Function is a block of code which performs a particular task and it is reusable. Functions can be called when necessary without implementing it again.

In Javascript functions can be created in three ways

Function Declarations
Function Expressions
Arrow Functions

Function Declarations

In Javascript function is created with the keyword function followed by function name

function addTwoIntegers(argument1, argument2) {
	var sum = argument1 + argument2;
	return sum;
}

Here the function name is addTwoIntegers and is performing addition of two integers. It takes two parameters argument1, argument2 and returns the output sum. Any code after the return statment will not be executed.

Function will not be executed until it is called. Above function can be called by the function name.

var number1 = 10;
var number2 = 20;

var output = addTwoIntegers(number1, number2);

console.log(output); // prints 30

Function Scope

Local Variables

Variables created inside the functions can not be accessed outside the function. Variables created inside the function have the scope limit within the function.

function addTwoIntegers(argument1, argument2) {
	var sum = argument1 + argument2;
	return sum;
}

addTwoIntegers(20, 30);

console.log(sum); // Uncaught ReferenceError: sum is not defined

Global Variables

Variables defined outside the functions can be accessed inside the functions. This helps to reuse the variables.

var sum = 0;

function addTwoIntegers(argument1, argument2) {
	sum = argument1 + argument2;
	return sum;
}

addTwoIntegers(20, 30);

console.log(sum); // prints 50

Default values for Parameters

If the parameters for the functions are not passed, it will throw an error. Missing parameter is considered as undefined

var sum = 0;

function addTwoIntegers(argument1, argument2) {
	sum = argument1 + argument2;
	return sum;
}

addTwoIntegers(20);

console.log(sum); // prints NaN

In Javascript we can assign a default value to a parameter.

var sum = 0;

function addTwoIntegers(argument1, argument2 = 0) {
	sum = argument1 + argument2;
	return sum;
}

addTwoIntegers(20);

console.log(sum); // prints 20

Function Expressions

The above Function Declaration is synatically correct, functions can also be created by a function expression.

var sum = function (argument1, argument2) {
	sum = argument1 + argument2;
}

sum(20, 30);

console.log(sum); // prints 50

Function name also can be provided to the expression, It helps in identifying and debugging function.

var sum = function addTwoIntegers(argument1, argument2) {
	sum = argument1 + argument2;
}

sum(20, 30);

console.log(sum); // prints 50

Arrow Functions

It is a more simple and concise syntax way for creating function.

var sum = (argument1, argument2) => argument1 + argument2;

console.log(sum(10, 20)); // prints 30

Arrow function can be used for functions having multi line

var sum = (argument1, argument2) => {
	var result = argument1 + argument2;
	return result // prints 30
}

These are user defined functions. Javascript itself has top level built in function, populary known as built in functions

Built in functions

Javascript has many built in functions which can be used without implementing again.

Examples:

eval() - Method evaluates the javascript code represented as string

isNaN() - checks if value is NaN

parseInt() - parses a first argument string and returns Integer.

encodeURI() - Encodes special charactes in the URI

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