Question

undefined is technically able to be redefined, so it is not a reserved word. As a result, I usually write code in an anonymous function that forces undefined to be an undefined variable, as so:

(function (undefined) {
    "use strict";
    var o = {
        test: "testvalue"
    };
    if (o.test === undefined) {
        // Do stuff here
    } else {
        // Do other stuff there
    }
}());

However, JSLint mentions the following error:

Problem at line 1 character 15: Expected an identifier and instead saw 'undefined' (a reserved word).

Why does JSLint complain about undefined being an reserved word, when code can arbitrarily redefine the variable? I know that you can use typeof x === "undefined"; I just wanted to see why this method wouldn’t work.

Answer 1

Your method does work. Just because JSLint doesn’t like it doesn’t make it a cardinal sin.

Try JSHint instead (for more sanity).