JavaScript is not so well structured like other languages (C#, Java). When you define a variable like this

var myVariable;

You do not specify type of variable. It can be string, object, array or other JavaScript type. So, passing variable to a function can a problem as very easy you can pass a type which is not accepted by the function. But, ASP.NET Ajax is offering a way to validate parameters. Consider the code below:

function DoSomething(expectedVariable) {
	var e = Function._validateParams(arguments, [{ name: "expectedVariable", type: String }], true);
 	if (e) {  throw e; }

Code above is checking is if parameter passed to DoSomething function is a string and stop code execution if condition is not true, throwing an error. Other types can be also checked: Object, Array, Function. You can also specify if parameter is option or if it can be null.

If you are asking from where Function._validateParams comes from, you can check ASP.NET AJAX JavaScript library. It was supposed to be used internally (“-“ sign, by convention, means property or method is private), but without no issues you can use it directly in your code. If you have version 4 of the library, Microsoft included also a public version of this called “Function.validateParameters”. Please find official reference here.