Mozilla and asm.js

Mozilla debuted asm.js in 2013 and provided a way for developers to translate their C and C++ source code to JavaScript. The official specification for asm.js defines it as a strict subset of JavaScript that can be used as a low-level, efficient target language for compilers. It's still valid JavaScript, but the language features are limited to those that are amenable to ahead-of-time (AOT) optimization. AOT is a technique that the browser's JavaScript engine uses to execute code more efficiently by compiling it down to native machine code. asm.js achieves these performance gains by having 100% type consistency and manual memory management.

Using a tool such as Emscripten, C/C++ code can be transpiled down to asm.js and easily distributed using the same means as normal JavaScript. Accessing the functions in an asm.js module requires linking, which involves calling its function to obtain an object with the module's exports.

asm.js is incredibly flexible, however, certain interactions with the module can cause a loss of performance. For example, if an asm.js module is given access to a custom JavaScript function that fails dynamic or static validation, the code can't take advantage of AOT and falls back to the interpreter: