Variables

Variables

Variables are used to store any type of JavaScript values.

To declare a variable, first write the var keyword. This tells that a variable is being created. It is then followed by the name of the variable.

var variable_name;

While declaring a variable, you can assign its value using equal (=) operator, followed by the value.

var variable_name = "value";

To declare multiple variables on a single line, write var keyword and separate each variable with a comma.

var fruit = "Apple", var2, some_number = 100;

Variable names can contain letters, digits, underscore (_) and dollar sign ($). The names must start with a letter, underscore or a dollar sign. Here are some examples:

var a = 9;
var $b = "hello";
var test$test = 50;
var _myVar = 1.2;
var employee_name1 = "john doe";

Javascript variables are case sensitive, so having variables named like var a; and var A; would be considered as being two separate variables.

var a = 9;
var A = 4;
document.write(a); //This displays 9

The above code will display the following:

9

If you declare a variable and not set its value, the variable will have value of undefined.

var name1;
document.write(name1); //This displays "undefined"
document.write("<br>"); 
name1 = "Homer J Simpson";
document.write(name1); //This displays "Homer J Simpson"

The above code will display the following:

undefined
Homer J Simpson

Variable Scope

Javascript variables have two scopes: global and local.

A variable declared outside a function has global scope it's value can be accessed throughout the program.

<script type="text/javascript"> 
    var a = 9;
    function Hello() {
        document.write(a); //This displays 9
    }
    Hello(); //Calling the Hello function 
</script>

The above code will display the following:

9

A variable declared inside a function has a local scope, it would only be accessible inside the function it was declared in. The local variable will take precedence over same named global variable.

<script type="text/javascript"> 
    var a = 9;
    function Hello() {
        var a = 6;
        document.write(a); //This will display 6
    }
    Hello(); //Calling the Hello function
    document.write("<br>");
    document.write(a); //This will display 9 
</script>

The above code will display the following:

6
9