variable === undefined vs. typeof variable === “undefined”

Question

The jQuery Core Style Guidelines suggest two different ways to check whether a variable is defined.

• Global Variables: typeof variable === "undefined&

Answer 1

Yet another reason for using the typeof-variant: undefined can be redefined.

undefined = "foo";
var variable = "foo";
if (variable === undefined)
  console.log("eh, what?!");

The result of typeof variable cannot.

Update: note that this is not the case in ES5 there the global undefined is a non-configurable, non-writable property:

15.1.1 Value Properties of the Global Object
[...]
15.1.1.3 undefined
The value of undefined is undefined (see 8.1). This property has the attributes
{ [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

But it still can be shadowed by a local variable:

(function() {
  var undefined = "foo";
  var variable = "foo";
  if (variable === undefined)
    console.log("eh, what?!");  
})()

or parameter:

(function(undefined) {
  var variable = "foo";
  if (variable === undefined)
    console.log("eh, what?!");  
})("foo")