Asm.js

Asm.js is a strict subset of JavaScript, that C/C++ code can be compiled to.

An asm.js compiled C/C++ code can run in any browser, because it is JavaScript and it is more performant, because it is a highly-optimized subset of JavaScript.

Asm.js is usually less performant than WebAssembly.

Asm.js is superseded by WebAssembly.

External links

https://en.wikipedia.org/wiki/Asm.js

https://johnresig.com/blog/asmjs-javascript-compile-target/

http://asmjs.org/

https://mrale.ph/blog/2013/03/28/why-asmjs-bothers-me.html?