V8 has a very nice profiler. Functions are the smallest granules it understands, but it's very good for finding slow functions, and (perhaps even more important

V8 has a very nice profiler. Functions are the smallest granules it understands, but it's very good for finding slow functions, and (perhaps even more importantly) for finding out which functions v8 has given up on optimizing. But I'm guessing it didn't help for this particular problem. Deleting properties from an object can hurt performance in v8, but it's not the deletion that's slow, it just makes the later accesses to that object slower. So the line of code that needed changing may not have been anywhere near the code that profiled poorly.