In theory yes, a smart enough JIT could figure this out. That was in fact v8's strategy for a while. But in practice, it's just not as effective. In a small fun

In theory yes, a smart enough JIT could figure this out. That was in fact v8's strategy for a while. But in practice, it's just not as effective. In a small function like that things are fairly simple, but with large code it takes time and resources for the optimizer to figure out all the types. That leads to noticeable slowness and pauses for recompilation etc. Instead of that, asm.js gives you the option to just receive the types up front.