Looks like I’m late to the party, but the trailing comma is a rather handy feature. Sure it looks hideous in a one-liner array like in your example, but consider it in an array where each element of the array is long enough to deserve its own line. Now consider the event when you re-order some of those lines, and one of those lines is the last one. Now you need to add a comma to the formerly last element, and remove the comma from the new last element. If you use a trailing comma, you don’t need to worry about this.

Also, a syntax that allows trailing commas makes automatic source code generation easier.