How does it relate to Emscripten?

Emscripten is the source-to-source compiler that can generate asm.js from C and C++ source code. We'll use it as a build tool to generate the Wasm modules. In this section, we'll quickly review how Emscripten relates to WebAssembly.

Emscripten's role

Emscripten is an LLVM-to-JavaScript compiler, which means it takes LLVM bitcode output of a compiler such as Clang (for C and C++), and converts that to JavaScript. It isn't one specific technology, but rather a combination of technologies that work together to build, compile, and run asm.js. To generate Wasm modules, we'll use the Emscripten SDK (EMSDK)  Manager:

Wasm module generation with the EMSDK

The EMSDK and Binaryen

In Chapter 4, Installing the Required Dependencies, we'll install the EMSDK and use it to manage the dependencies required to compile C and C++ to Wasm modules. Emscripten uses Binaryen's asm2wasm tool to compile the asm.js output by Emscripten to a .wasm file. Binaryen is a compiler and toolchain infrastructure library that includes tools to compile various formats to WebAssembly modules and vice versa. Understanding the inner workings of Binaryen isn't required to use WebAssembly, but it is important to be aware of the underlying technologies and how they work together. By passing certain flags into the compile command for Emscripten (emcc), we can pipe the resultant asm.js code to Binaryen to output our .wasm file.