2010-02-15-undefined-is-mutable.md

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