(Note: I worked on SpiderMonkey's JIT and various optimization-related things) In general the accepted "best practice" approach is always to use the length of

(Note: I worked on SpiderMonkey's JIT and various optimization-related things) In general the accepted "best practice" approach is always to use the length of the compiled bytecode as a heuristic. This is what we use basically everywhere in the JIT for "function size" heuristics - inlining logic being the biggest user. V8 doesn't have a bytecode (although I've heard they're adding a bytecode interpreter now), but even there, they can use AST-weight (i.e. the count of all vertices in the AST for a function) as a measure of the function size. Counting bytes, characters, or any sort of raw source-related metric is a bad idea. These things happen because it's easy to use an easy heuristic that'll work most of the time. The size of a function in source-code bytes is easy to get, and always available. You may not be able to guarantee that bytecode or AST info is available when you want to use it (e.g. if the function has been lazified due to lack-of-use and all the compiled bits thrown away).