JS Variables

JS variables are basically containers for storing information. As with Algebra in Mathematics, JS variables are used to hold values or expressions. A variable can have a short name, like “num”, or a more descriptive name, like “string”.

Rules for naming of JS variables:
  • JS variable names are case sensitive (‘p’ and ‘P’ are two different variables)
  • JS variable names must begin with a letter or the underscore character

N.B. As because JS is case-sensitive, variable names are also case-sensitive.

Example. A variable’s value can change during the execution of a script. We can refer to a variable by its name to display or change its value. This example will show us how to do this:

<script type="text/javascript">
var firstname;
<p>The script above declares a variable, assigns a value to it, displays the value, change the value,
and displays the value again.</p>


Creating JS Variables

Creating variables in JS is most often referred to as declaring variables. We can declare JS variables with the var statement:

var num;
var string;

After the declaration shown above, the variables are empty (they have no values yet). However, we can also assign values to the variables when we declare them:

var num = 10; 
var string = "motherland";

After the execution of the statements above, the variable num will hold the value 10, and string will hold the value motherland.

N.B. When we assign a text value to a variable, use double quotes around the value.

Assigning Values to Undeclared JS Variables:

If we assign values to variables that have not yet been declared, the variables will automatically be declared. These statements are:

num = 10;
string = "motherland";

have the same effect as:

var num = 10;
var string = "motherland";


Re-declaring JS Variables:

If we re-declare a JS variable, it will not lose its actual value.

var num = 10;
var num;

After the execution of the JS statements above, the variable num will still have the value of 10. The value of num is not reset when we re-declare it.


JS Arithmetic:

Like algebra in mathematics, we can also perform arithmetic operations with JS variables:

b = a - 10;
c = b + 10;