Computer Consultant

Do You Remember Algebra From School?

Hopefully you remember algebra like this from school: x=5, y=6, z=x+y.

Do you remember that a letter (like x) could be used to hold a value (like
5), and that you could use the information above calculate the value of z to be
11?

Sure you do!

These letters are called variables, and variables can be used to hold values
(x=5) or expressions (z=x+y).

JavaScript Variables

As with algebra, JavaScript variables are used to hold values or expressions.

A variable can have a short name, like x, or a more describing name
like length.  

A JavaScript variable can also hold a text value like in carname="Volvo".

Rules for JavaScript variable names:

• Variable names are case sensitive (y and Y are two different
  variables)



• Variable names must begin with a letter or the underscore character

NOTE:  Because JavaScript is case-sensitive, variable names are
case-sensitive.

Example

A
variable's value can change during the execution of a script. You can refer to a variable by
its name to display or change its value.

This example will
show you how

Declaring (Creating) JavaScript Variables

Creating variables in JavaScript is most often referred to as "declaring"
variables.

You can declare JavaScript variables with the var statement:

var x;
var carname;

After the declaration shown above, the variables has no values, but you can assign values to the variables while you declare them:

var x=5;
var carname="Volvo";

Note: When you assign a text value to a variable, you use quotes
around the value.

Assigning Values to JavaScript Variables

You assign values to JavaScript variables with assignment statements:

x=5;
carname="Volvo";

The variable name is on the left side of the = sign, and the value you
want to assign to the variable is on the right.

After the execution of the statements above, the variable x will hold
the value 5, and carname will hold the value Volvo.

Assigning Values to Undeclared JavaScript Variables

If you assign values to variables that has not yet been declared,
the variables will automatically be declared.

These statements:

x=5;
carname="Volvo";

have the same effect as:

var x=5;
var carname="Volvo";

Redeclaring JavaScript Variables

If you redeclare a JavaScript variable, it will not lose its original value.

var x=5;
var x;

After the execution of the statements above, the variable x will still have
the value of 5. The value of x is not reset (or cleared) when you redeclare it.

JavaScript Arithmetic

As with algebra, you can do arithmetic with JavaScript variables:

y=x-5;
z=y+5;

You will learn more about the operators that can be used between JavaScript
variables in the next chapter of this tutorial.