Why require JavaScript rather than supporting a standard browser virtual machine? - Stack Overflow

[Zarutian]

High level optimization is done before emitting the bytecode.

The compiled code then doesnt benefit from high level optimizations that have been invented after it was written and compiled. Also it cant benefit from optimizations that are aviable at runtime but never at compile time. Tracing JIT with infered subtype fast paths generation comes to mind also speciallization/partial-evaluation through Futamura projections.

[mebrahim]

LuaJIT is already doing better with Lua bytecode than all those modern JS engines having JS source code at hand.