In JavaScript, undefined is nothing but a global variable name without a default value. Therefore, its primitive value is undefined. You can change the value of undefined:
var a = {};
a.b === undefined; // true because property b is not set
undefined = 42;
a.b === undefined; // false
Due to the mutability of undefined, it is generally a better idea to check for undefined-ness through typeof:
var a = {};
typeof a.b == 'undefined'; // always true
– @mathias