[djgrant]

After 15 minutes - http://jsfiddle.net/NSdyg/ Another 5 to make it work (almost) - http://jsfiddle.net/NSdyg/1/

I don't do much JS (as you can likely tell) but found the task approachable. I think that the low barrier to entry is a good thing for an interview question.

My approach is described in the comments. I started by writing it out and then went from there.