V8 is a JIT compiler that uses the Ignition [1] interpreter and only compiles sections of code down to machine instructions once they've been marked as hot via TurboFan [2].

V8 can also go back and forth from machine instructions back to bytecode if it identifies that certain optimization assumptions no longer hold.

[1] https://v8.dev/docs/ignition

[2] https://v8.dev/docs/turbofan

That literally is the definition of JIT, it does a quick parse, compiles hot parts and improves it later on