[jpsamaroo]

JavaScript's JIT is a tracing JIT, so it can compile code in the background while the interpreter/less optimized compiled code is actually running. In Julia, the compiler runs first, and then the compiled code is run. This will probably eventually change as Julia's compiler improves, but regardless, it's important to note this distinction.

[pjmlp]

It can make use of JIT caches (if it doesn't already), not every JIT compiler starts from zero.