web platform can be thought of as having two parts:
A virtual machine (VM) that runs the Web app’s code, e.g. the JavaScript code that powers your apps.
A set of Web APIs that the Web app can call to control web browser/device functionality and make things happen (DOM, CSSOM, WebGL, IndexedDB, Web Audio API, etc.).
WebAssembly is a different language from JavaScript, but it is not intended as a replacement. Instead, it is designed to complement and work alongside JavaScript,
WebAssembly is a low-level assembly-like language with a compact binary format that runs with near-native performance and provides languages with low-level memory models
Emscripten implements popular C/C++ libraries like SDL, OpenGL, OpenAL, and parts of POSIX. These libraries are implemented in terms of Web APIs and thus each one requires some JavaScript glue code to connect WebAssembly to the underlying Web API.
Most languages provide a standard library. While coding, the programmer doesn’t need to know what system they are targeting. They just use the interface
system interface comes in. For example, printf being compiled for a Windows machine could use the Windows API to interact with the machine. If it’s being compiled for Mac or Linux, it will use POSIX instead
Emscripten created its own implementation of libc. This implementation was split in two — part was compiled into the WebAssembly module, and the other part was implemented in JS glue code. This JS glue would then call into the browser, which would then talk to the OS.
when people started wanting to run WebAssembly without a browser, they started by making Emscripten-compiled code run.
So these runtimes needed to create their own implementations for all of these functions that were in the JS glue code.
WebAssembly is as an alternative ‘backend’ for Docker tooling. You can use the same command line tools and workflows, but instead of using Linux containers, it is implemented using WebAssembly-based container equivalents.
another tool called Emscripten which is a bit easier to use at the moment. It has its own back-end that can produce WebAssembly by compiling to another target (called asm.js) and then converting that to WebAssembly. It uses LLVM under the hood, though, so you can switch between the two back-ends from Emscripten.
