Question

This is weird. Check this out:

 for( var i = 0; i <= videos.length; i ++ ){

    alert(videos[i].id); // this works and alerts the correct number

    var foo = videos[i].id; // firebug says "videos[i] is undefined"

 }

There are 3 videos. In FF this alerts all 3 video id’s then fails saying that videos[i] is undefined. No Idea, at all.

Answer 1

Get rid of the = in your for loop condition.

for( var i = 0; i < videos.length; i ++ ){
    ...
}

With <=, you are iterating to the index, one larger than the actual index value of the Array, so you are iterating over an invalid index which returns undefined.

For example…

If you have array('A','B','C'), the length is 3. Now, if you iterate to 3 <= i, and include 0, as arrays begin with in Javascript, you will actually loop 4 times, not three.

The index value of A is 0, not 1, so you need to stop BEFORE i is equal to the length, not continue until i is equal to the length, since the 0 index is essentially added to the total length for the looping, meaning 3+1. 4 loops over this array would be one too many, hence the < and not <=. You want to STOP before 4, not stop AFTER 4 but before 5.

Also, it is generally good practice to cache the length of the Array, because some browsers do not optimise it.