I ran some performance tests on some of the cases (as well as my own solution). The top solution using reduce() performs the worst. The best solution is the one using modulus to loop through the numbers. I was actually surprised my solution did as well as it did since I used Math.pow() which I know can be expensive.

http://jsperf.com/sum-digits-of-a-number

Is code efficency even matter? No wonder most websites are so slow.