Optimizations like 'these two function arguments are always int31' in v8 or spidermonkey are 100% predictable at this point and result in all your type checks a

Optimizations like 'these two function arguments are always int31' in v8 or spidermonkey are 100% predictable at this point and result in all your type checks and boxing being eliminated, and with the known types it also becomes much cheaper/faster to create object instances (since now if you store those values into properties of an object, that object's shape is fully known). Various properties like this can extend out into larger parts of your JS application. There's still a lot of magic you can't rely on, but you'd be surprised how much you CAN rely on. Asm.js was built on this observation: If you write your JS following some basic rules it's actually pretty easy to land on predictable, well-optimized paths. Of course, one of WASM's advantages is that by design you're almost always on those paths and don't have to worry.