WASM
WASM stands for Web Assembly. Another topic that's confusing when we take the name literally.
It's not a programming language. WASM means assembly language for the web. It's a low-level assembly like language that runs in the browser.
The main goal of WASM was to run code that's already written in different languages in browser. There was no point in porting the entire code to JS. WASM compiler instead took the original code and generated WSAM code for it.
Browser then provided two runtimes. One for JS and one for WASM.
WASM Compiler and Engine
WAS Compiler generates code that's already assembly like. This is why it's called assembly and near-native.
The engine does minimal work to generate the native code at runtime.
When executed, WASM is only an engine since it only knows how to parse WSAM files. The engine only runs computational code. Anything else such as for example., loading a file must come from the host environment.
This is exactly why we say WASM is a sandbox by default.
Host environment is the parent runtime that runs the WASM engine. For example, in a browser, the host environment is the browser itself. It provides all other functions that the WASM compiled code needs.
Using Host Functions
It's important to understand how the host functions exchange data with the WASM engine. It happens all via the memory.
When the WSAM engine is instantiated, all the allowed functions of the host is added to a list that's passed to the engine's constructor.
These are nothing but the function references. When the function is called, the engine knows that it's the scope of the host and pauses. The engine then schedules the host function in the instruction pointer to run.
QuickJS
QuickJS is what makes things even more interesting and confusing.
QuickJS is a JS runtime. It can parse JS code and execute. It's written in C and compiled to WASM. Meaning, we pass the actual JS code to QuickJS, and it will execute it for us inside the WASM sandbox.