SpiderMonkey caches pre-parsed bytecode for scripts, which can immediately start executing in the interpreter. (I believe roughly the same is true for other eng

SpiderMonkey caches pre-parsed bytecode for scripts, which can immediately start executing in the interpreter. (I believe roughly the same is true for other engines. See [1], for example.) The tricky part of caching compiled scripts is that they generally refer to various runtime state. For example, a compiled script might include a check to verify the shape / hidden class of an object. If you want to cache that code, you also have to cache the shape tree. Multiply that out by all the runtime state that is referenced from compiled code, and it's a hard problem. We've talked about simpler mechanisms like caching function hotness / the number of iterations before our profiling data stabilizes, to trigger compilation earlier, but we haven't gotten around to implementing it yet. [1]: https://v8.dev/blog/code-caching-for-devs https://v8.dev/blog/code-caching-for-devs