Asm.js

asm.js is a subset of JavaScript designed to allow computer software written in languages such as C to be run as web applications while maintaining performance characteristics considerably better than standard JavaScript, which is the typical language used for such applications.

asm.js consists of a strict subset of JavaScript, to which code written in statically typed languages with manual memory management (such as C) is translated by a source-to-source compiler such as Emscripten (based on LLVM). Performance is improved by limiting language features to those amenable to ahead-of-time optimization and other performance improvements.

Mozilla Firefox was the first web browser to implement asm.js-specific optimizations, starting with version 22.

asm.js is superseded by WebAssembly.