[fenomas]

Having worked on this pretty extensively, my preferred method is: (1) make two implementations of the function I'm trying to speed up, (2) add a wrapper so half of that function's calls get sent to each version, and then (3) profile in Chrome and compare the results (particularly the time spent in the two alternate implementations, but overall as well).

I know that's not the gdb-like experience you're looking for, but as far as I've found it's the best approach. The issue is that modern JS engines achieve their speed by tracking what happens at runtime and dynamically re-optimizing hot functions - so microbenchmarks are largely meaningless, and the performance of a given function can hugely affected by code that's far away.

(The above is for everyday. For extreme deep-diving, one can use JS engine tools to see what kind of internal representation your code has been compiled into after it got optimized. In chrome this is done with IRHydra - http://mrale.ph/irhydra/2/ .)

[Kholo]

Thanks for link. Didn't know this was possible. Also will try your wrapper approach.