This page had 9 tips for improving your Javascript.
One of them struck me:
2. String Concatenation vs. Array.join
(Array.join is faster in Javascript)
Curious, I wrote a simple program to exercise the same functionality in ActionScript 3.
It turns out that the opposite is true. For the test I did, concatenanting 100,000 strings together, String concatenation was 3x faster than Array.join