The reason most of the features of the JIT are needed is due to the lack of static typing in javascript. The most important thing the JIT does is figure out wha

The reason most of the features of the JIT are needed is due to the lack of static typing in javascript. The most important thing the JIT does is figure out what types of things you are using and optimize for them. There will always be some cost here, because at runtime you need to check if your assumptions still hold. Some of the time you can pull these checks out of loops, but sometimes you can't, and they can be costly. There are a few things a JIT can theoretically do better. Inlining is the biggest one. For example, a JIT knows not to inline a function that never gets called, and it can also do inlining for indirect calls based on profile data. A JIT could also theoretically move unexecuted blocks out of line for better instruction cache locality. It could also check for aliasing at run time (with a bailout) and then optimize a loop assuming no aliasing. Unfortunately for JITs, AOT compilers can do a pretty good simulation of most of these things with PGO, that is running a scenario with an instrumented binary and re-optimizing using profile data. Still can't bailout if an assumption is wrong, so it doesn't work for dynamic languages, but you don't really need bailout for much in static languages.