Do You Remember Algebra From School?

Do you remember algebra 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 to calculate the value of z to be 11?

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 descriptive name, like carname.
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.

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 declare JavaScript variables with the var keyword:
var x;
var carname;
After the declaration shown above, the variables are empty (they have no values yet).

However, you can also assign values to the variables when you declare them:
var x=5;
var carname="Volvo";
After the execution of the statements above, the variable x will hold the value 5, and carname will hold the value Volvo.

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

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

Local JavaScript Variables

A variable declared within a JavaScript function becomes LOCAL and can only be accessed within that function. (the variable has local scope).

You can have local variables with the same name in different functions, because local variables are only recognized by the function in which they are declared.

Local variables are destroyed when you exit the function.

Global JavaScript Variables

Variables declared outside a function becomes GLOBAL, and all scripts and functions on the web page can access it.

Global variables are destroyed when you close the page.

If you declare a variable, without using "var", the variable always becomes GLOBAL.

Assigning Values to Undeclared JavaScript Variables

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

These statements:
will declare the variables x and carname as global variables (if they don't already exist).

JavaScript Arithmetic

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