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:
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.
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.
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.
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:
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 |