That specific version is @michaelficarra suggestion: https://github.com/eslint/eslint/pull/529#discussion_r8986074 - though my initial version also used bitwise (https://github.com/dmp42/eslint/commit/73f40871eca5e36be1770f7d61e120dbdffa4af6) in a while.
Now, yours as above won't work (you want var result = ""; result += str instead). Ultimately I don't care which one gets in - they all work - the bitwise versions are faster, and look better IMHO, but it's not like this is a perf critical path, or that (my) personal taste matter...