Modern JS VMs have type-specializing JITs. That's how they make code run fast, basically - they detect types at runtime, and emit efficient code for that. asm.

Modern JS VMs have type-specializing JITs. That's how they make code run fast, basically - they detect types at runtime, and emit efficient code for that. asm.js just gives them the types, which makes things easier, but it all builds upon the existing power of JS VMs. See e.g. http://kripken.github.io/mloc_emscripten_talk/sotu.html#/16 http://kripken.github.io/mloc_emscripten_talk/sotu.html#/16