How dangerous is it in JavaScript, really, to assume undefined is not overwritten?

Question

Every time anyone mentions testing against undefined, it\'s pointed out that undefined is not a keyword so it could be set to \"hello\", so you sho

Answer 1

You can use undefined in your code when coding for browsers supporting ECMAScript 5.1 as it is immutable according to the language specification.

Also see this compatibility table or this caniuse ECMAScript 5 to see that all modern browsers (IE 9+) have implemented immutable undefined.