Why should I use underscore's isUndefined(x) over x === undefined? [closed]

Question

Is there any benefit in using isUndefined? Is it worth an extra function call? It's not any more readable.

Answer 1

The name undefined can be shadowed. That is, somebody could do this

var undefined = 5;

and break the code that uses x === undefined (see note at bottom). To get around this safely, you can use

typeof x === 'undefined'

or

x === void 0

which is exactly what the underscore function does.

---

Note: Since ECMAScript 5, undefined is read-only. In older browser, the global undefined can be redefined. Even in newer browsers, undefined can be shadowed by a local variable:

function f() {
  var undefined = 5;
  return undefined;
}
f() // returns 5