IE9 JavaScript array initialization bug

Question

Apparently JS implementation in IE9 contains (IMO, critical) bug in handling array literals.

In IE9 in some cases this code:

var a = [1,2,3,4,];
         


        

Answer 1

That's not a bug. That's exactly how it should behave. Microsoft did that on purpose. If you want an array with only 4 items, get rid of the last comma. Simple as that.

If the results you're after is to have an extra, undefined value at the end, you're in luck. Even without the comma, it'll be undefined. It, and every single number after 3.