when i was a full-time hobbyist webdev i was surprised when i learned c compilers don't do whole-program optimization by default but limit themselves by optimizing each translation unit separately (i guess it makes things simpler)

you can get "LTO" in javascript by using a minifier together with something like webpack which can inline modules when it's safe to do so (instead of putting them in functions that are called by the module loader and can't be minified away), so i assumed "real" compilers must do that stuff a lot better with the language being less dynamic