asm.js subset is basically JavaScript where you can only use numbers (no strings, objects, etc.). This is all you need to run C++ code since everything in C++ is either a number or a pointer to a number, and pointers are also numbers. The C++ address space is just a giant JavaScript array of numbers and pointers are just indices into that array.

We standardize on a finite subset of JS (such as asm.js) — and avoid the endless struggle through future iterations of the JavaScript language, competing super-sets and transpilers