Declaring variables within JScript is pretty easy using a simple VAR statement with the name of the variable you want to declare. For example, to declare a variable called invoice_total, your declaration statement would look like this:
var invoice_total;
The name and case of your variable is important and is one of the main errors you will encounter when working with JScript. You can name variables using a mix of alphanumeric characters, underscores, and dollar signs, but the variable name has to begin with a letter, underscore, or dollar sign. In addition, you can’t name a variable the same name as a reserved word (a list of reserved words in JScript is provided near the end of this appendix).
When working with variables, six different types can be associated with a variable:
number
string
object
Boolean
null
undefined
When a variable is first created and unassigned (in other words, there is no value assigned to it) it is considered to be undefined:
var sales_tax;
When an assignment is made to the variable, it then takes the type of the value assigned. For example:
var sales_tax = 10; //Number
var sales_tax = "Not applicable"; //String