Arrays do seem to be the way to make Chrome happy, although unfortunately they're slower for everything else:

http://jsperf.com/backreference-copy

I don't see why you say an array is a more appropriate datastructure for the algorithm, though--that seems very much implementation dependent. And at the JS level, arrays support operations such as splice that strings do not, suggesting that they'd typically be implemented with a more sophisticated datastructure, and hence slower at primitive operations like index access. (Perhaps V8 notices that these expensive operations aren't actually used, and creates a fast, basic version instead.)