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JavaScript Variables

A JavaScript variable is nothing but a name of storage location. Like other programming languages, JavaScript has variables, you can assign a value to a variable when you declare it or before using it. JavaScript uses a reserved keyword var to declare a variable.

Syntax:

var variable-name;
var variable-name = value;

Example:

var one = 5; // variable stores numeric value
var two = 'six'; // variable stores string value
var three; // declared a variable without assigning a value
Try it
In the above example, we have declared three variables using var keyword: one, two and three respectively. After that, we have assigned values to variables one and two at the time of declaration, whereas variable three is declared but the value is not assigned to it, so its value will be ‘undefined’.

Declare Variables in a Single Line:

Multiple variables can also be declared in a single line separated by a comma.

Example:

var one = 1, two = 'two', three;
Try it

Declare a JavaScript Variable without var Keyword:

JavaScript allows us to declare a variable without var keyword. A value must be assigned to a variable when you declare a variable without var keyword.

Example:

one = 1;
two = 'two';
Try it
Note: It is not recommended to declare a variable without var keyword because it can accidentally overwrite an existing global variable.

Rules for declaring a JavaScript variable:

  • A variable name must start with a letter (a to z or A to Z), underscore( _ ), or dollar( $ ) sign.
  • We can use digits (0 to 9), after the first letter.
  • JavaScript variables are case sensitive, for example, a and A are different variables.

JavaScript Variable Scope:

The region of your program in which variable is defined is called scope.
JavaScript variables have only two scopes.
  • Global Variables
  • Local Variables

Global Variables:

A global variable has a global scope and can be defined anywhere in your JavaScript code.

Example:

var demo = "HTML";
// code here can use variable demo
function myFunction() {
// code here can also use variable demo
}
Try it

Local Variables:

A local variable has a local scope which means it will be visible only within a function where it is defined. Function parameters are always local to that function.

Example:

// code here can use variable demo
function myFunction() {
var demo = "HTML";
// code here can use variable demo
}
Try it

Reserved Words in JavaScript:

Following is the list of the reserved word in JavaScript and they cannot be used as JavaScript variables, functions, methods or object names.
abstract else instanceof switch
boolean enum int synchronized
break export interface this
byte extends long throw
case false native throws
catch final new transient
char finally null true
class float package try
const for private typeof
continue function protected var
debugger goto public void
default if return volatile
delete implements short while
do import static with
double in super

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