Answered in asm.js FAQ: "Q. Why not just keep optimizing JavaScript JIT compilers instead? A. No need to stop! But JIT compilers have less predictable perform

Answered in asm.js FAQ: "Q. Why not just keep optimizing JavaScript JIT compilers instead? A. No need to stop! But JIT compilers have less predictable performance based on complicated heuristics. The asm.js model provides a model closer to C/C++ by eliminating dynamic type guards, boxed values, and garbage collection. Q. Isn't it inefficient to have to run the code through a JavaScript interpreter before compiling? A. Because of the directive prologue, a JavaScript engine can immediately recognize asm.js code at compile-time and immediately compile it to assembly language, with no need to ever run the code through an interpreter."