Javascript Strict Mode

Strict mode is a way to introduce better error-checking into your code. When you use strict mode, you cannot, for example, use implicitly declared variables, or assign a value to a read-only property, or add a property to an object that is not extensible. The restrictions are listed in the Restrictions on Code in Strict Mode section later in this topic.

You can declare strict mode by adding "use strict"; at the beginning of a file, a program, or a function. This kind of declaration is known as a directive prologue. The scope of a strict mode declaration depends on its context. If it is declared in a global context (outside the scope of a function), all the code in the program is in strict mode. If it is declared in a function, all the code in the function is in strict mode.

"use strict";
function testFunction(){
    var testvar = 4;
    return testvar;
}

// This causes a syntax error.
testvar = 5;

http://msdn.microsoft.com/en-us/library/br230269(v=vs.94).aspx