Testing for Javascript undefined correctly

I believe there are a number of incorrect answers to this topic. Contrary to common belief, "undefined" is NOT a keyword in JavaScript and can in fact have a value assigned to it.

// DO NOT USE the following bad examples, they are degenerate!

var undefined = false;  // Shockingly, this is completely legal!

 

var myVal;

if (myVar === undefined) {

    alert("You have been misled. Run away!");

}

if (myVar == undefined) {

    alert("You have been misled. Run away!");

}

 

Additionally, myVar === undefined will raise an error in the situation where myVar is undeclared.

The most robust way to perform this test is:

if (typeof myVar === "undefined")

 

This will always return the correct result, and even handles the situation where myVar is not declared.