Array Out of Bounds: Comparison with undefined, or length check?

You can also write:

if (index in array) {

which will return true even if array[index] is set to undefined.

Do not test for undefined. You should use the length of the array. There are cases where it simply does not work to test for undefined because undefined is a legal value for legitimate array entry. Here's a legal JS array:

var legalArray = [4, undefined, "foo"];

And you can access it like this:

var legalArray = [4, undefined, "foo"];

var result = "";
for (var i = 0; i < legalArray.length; i++) {
    result += legalArray[i] + "<br>";
}

$("#result").html(result);

Generates this output:

4
undefined
foo

As seen in this jsFiddle: http://jsfiddle.net/jfriend00/J5PPe/

The only correct way is to check the index vs. the length.

An element may be assigned the value undefined. It is just silly to use it for a sentinel here. (There may be other, valid and possibly overlapping, reasons for checking for undefined, but not "for an out of bound check" -- the code in the other question will present arguably wrong results when the value of the given arg is really undefined.)

Happy coding.