Technically, all edge cases considered, `typeof` is the only fool-proof check, since you can't override the string constant `'undefined'` but the `undefined` id

Technically, all edge cases considered, `typeof` is the only fool-proof check, since you can't override the string constant `'undefined'` but the `undefined` identifier is fair game. Here is one such edge case: (function () { var undefined = true; return (function foo(x) { return x === undefined; }()); }()); // false Language pedantry aside, browsers these days will freeze `window.undefined` so you'd have to shoot yourself in your own scope ;)