asm.js is, at its heart, nothing more than a compiler hint. It tells the runtime that the JavaScript you're writing in this function follows some conventions that happen to be easy to optimize, but it's still JavaScript code. A browser that doesn't support asm.js will pause for a few nanoseconds to wonder what the heck that strange string at the start of the function is, then go on to execute the JavaScript code the same way it always does, with the same result as if it had known what asm.js is.
