For what it's worth, as far as I can tell SpiderMonkey is still more or less optimizing away the makeKeyConcat / makeKeyConcatObj testcases in Firefox 29, and a

For what it's worth, as far as I can tell SpiderMonkey is still more or less optimizing away the makeKeyConcat / makeKeyConcatObj testcases in Firefox 29, and all of them on trunk. I bet it's inlining the functions, discovering the arguments are constant strings and hence the return values are constant, constant-folding the if conditions, etc... I tried to work around that in http://jsperf.com/makekey-concat-vs-join/11 http://jsperf.com/makekey-concat-vs-join/11 but clearly that's not good enough. Microbenchmarking a decent optimizing compiler is hard; it will typically be able to figure out that the ubench is pointless and optimize it all away...