actually, asm.js is just javascript. Basically, Mozilla looked at what sort of javascript code that the different JIT's allready handle really well, and made a specification out of it. So even in Chrome, asm.js will run very efficiently. Mozilla figuered out a way to write javascript code that made type-information easy to extract, which again makes it easy to AOT-compile. For instance, the following code:

    function asmjs(i) {
        i = i|0;
        return (i + 1)|0;
    }

is valid javascript, and you can easily write this in your own programs. The "|0" means that the variable will be converted to a integer, because it is specified in the javascript standard. As an optimization, you can use this as a type annotation, kind of like writing "int i = 0;" This is what asm.js is in a nutshell, and why it's so easy to implement a special compiler for it.