When using the JavaScript or JScript engines, if a variable is declared without assigning a value to it, and the data type is never declared, then the default value will be undefined.
To determine if a variable or object property exists, you can check if its type is 'undefined' (which will work for both undeclared and declared variables and properties, and with both JScript and JavaScript):
if(typeof(x) == "undefined") { <some code> }
In JScript, if a variable is declared with a type, but without assigning a value to it, then the variable will assume the default value for the data type. For example, the default value for a numeric type is zero, and the default for the String data type is the empty string. However, if a variable is declared without a specified data type, then it will have an initial value and type of undefined, similar to JavaScript.