This is a decision a compiler can never make in a reliable way because it entirely depends on the actual input and is not known until runtime. You may get away

This is a decision a compiler can never make in a reliable way because it entirely depends on the actual input and is not known until runtime. You may get away with dynamic recompilation when you realize that the input is not what you assumed when you compiled the code but I really doubt that this is a smart and efficient way to go about it. And asm.js is no evidence for the no side - the information is in the original source code, it does matter and asm.js works around the JavaScript limitation and makes this information available to the JavaScript compiler.