Deviation between build-time and run-time is...normal. The fact that JS is a dynamic language that doesn't necessarily require a build step, but we've introduced one anyways, doesn't really change the equation.
It's still exactly the same problem as building a C codebase versus running it.
The reason the FE JS ecosystem is confusing is because there's this absurd pretense that they're doing something novel whenever they implement a feature or tooling that has existed for decades (just not for JS). And then, because its novel, they come up with new names (or misuse old names, like here) because they either intentionally or unintentionally avoid using the 1:1 mapping with standard tooling.
Just-In-Time for the build just doesn't make sense -- a JIT is a very specific thing; it compiles code at runtime. The term exists specifically to differentiate from compilation at build-time. If you make a JIT for build-time, you've lost all meaning
This is just rebuilding the code more often... through a filewatcher.
