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In JavaScript there are 2 types of variables
1. Local variables
2. Global variables

JavaScript local variables : Local variables are the variables declared with in a function. These variables have local scope meaning these are available only inside the function that contains them. Local variables are created when a function starts, and deleted as soon as the function completes execution.

function helloWorld()
{
var greeting = “Hello”;
// The variable greeting is available in the function
greeting = greeting + ” JavaScript”;
alert(greeting);
}

helloWorld();

// The variable greeting is not available outside the function
// Error : ‘greeting’ is undefined
alert(greeting);

JavaScript global variables : Global variables are the variables declared outside a function. Global variables have global scope meaning all scripts and functions on the page can access them. The lifetime of a global variable starts with it’s declaration and is deleted when the page is closed.

// Global variable
var greeting = “Hello”;

function helloWorld()
{
// The variable greeting is available in the function
greeting = greeting + ” JavaScript”;
alert(greeting);
}

helloWorld();

If you assign a value to a variable that has not been declared, it will automatically become a global variable, even if it is present inside a function.

function helloWorld()
{
// The variable greeting is not declared but a value is assigned.
// So it will automatically become a global variable
greeting = “Hello JavaScript”;
}

helloWorld();

// Variable greeting is available outside the function
alert(greeting);

A local variable can have the same name as a global variable. Changing the value of one variable has no effect on the other. If the variable value is changed inside a function, and if a local version of the variable exists then the local variable gets modified. If the variable value is changed outside a function then the global variable gets modified.

var greeting = “This is from global Variable”;

function helloWorld()
{
var greeting = “This is from local variable”;
document.write(greeting + “[br/]”);
}

// This line will modify the global greeting variable
greeting += “!!!”;

helloWorld();

document.write(greeting);

Output :
This is from local variable
This is from global Variable!!!

Sometimes, variable hoisting and local & global variable with the same name can cause unexpected behavior.

var greeting = “This is from global Variable”;
helloWorld();

function helloWorld()
{
document.write(greeting);
var greeting = “Hello from local variable”
}

Output :
undefined

At runtime due to variable hoisting, the above program would look more like as shown below.

var greeting = “This is from global Variable”;
helloWorld();

function helloWorld()
{
var greeting;
document.write(greeting);
greeting = “Hello from local variable”
}

Braces do not create scope in JavaScript : In the following example otherNumber is a global variable though it is defined inside braces. In many languages like C# and Java, braces create scope, but not JavaScript.

var number = 100;

if (number ] 10)
{
var otherNumber = number;
}

document.write(otherNumber);

Output : 100

Original source