It's a compact way to round a number without without needing to check for negatives and do Math.ceil/floor accordingly. ~~ is clever and useful but not intuitive :) http://stackoverflow.com/questions/4055633/what-does-do-in-javascript has a good writeup of how it works.

num+'' was my first thought, and as you say it's legit... but I went with toString() for readability - ie. you can be sure everyone on your team can work out what "toString" is doing, but the other way requires slightly more knowledge of implicit type coercion. To say nothing of this solution... impressive, but your team mates may not appreciate it ;)