asm.js
asm.js is a strict subset of JavaScript designed to serve as a low-level, efficient target language for compilers, particularly for C/C++ code. It enables near-native performance by using only a limited set of JavaScript features that can be heavily optimized by JavaScript engines. This allows developers to run performance-critical applications, such as games or simulations, in web browsers without plugins.
Developers should learn asm.js when they need to port existing C/C++ codebases to the web while maintaining high performance, as it provides a straightforward compilation path with minimal overhead. It is particularly useful for game development, scientific computing, and multimedia applications where WebAssembly is not yet supported or requires fallback compatibility. However, with the rise of WebAssembly, asm.js is now primarily relevant for legacy systems or as a polyfill.