• are case-sensitive. x and X are different variables
• must start with a letter, underscore, or dollar sign
• can be of any length
• have no limit on length
• cannot use reserved words

• string
  Example: "abc" or "123"

• number
  Example: 56 or 1.35

• boolean
  Example: true or false (only two boolean values are allowed)

• object
  (see example in Object tutorial)

The data type contained within a variable can be checked using the 'typeof' function provided by JavaScript, as in this example:

if (typeof myvariable="number") ...

Special Values
JavaScript also supports two special values which can be assigned to variables:

• Undefined - variable has been created but no value assigned
• Unll

Although JavaScript variable do not have to be declared before using them, it is considered good practive to do so.

To create a variable by assignment, use this code:

newVariable = 5		//create a variable simply by assigning it
newVariable = "dog"	//variables can be any type
newVariable = 'dog'	//single or double quotes can be used on strings

To create one or more variables by declaration, use the 'var' keyword, as in this example:

var X
var X, Y	//note the comma between the two variables

A variable can be assigned a value at the same time as it is created, as in this example:

var X = 6
var X=6,Y=7    //note the comma between the two declarations

Use of the 'var' operator affects the scope of the variable. Scope defines where a variable may be accessed within a JavaScript program. When 'var' is used to declare a variable, the variable may not be accessed within any function or other JavaScript program. It may only be accessed within the body of the JavaScript program where it was created.

Likewise, declaring a variable using 'var' within a function denies access to any JavaScript code outside that function.

See the Function tutorial for more information on Scope.