We use Web Workers as the basis for our processes, and processes perform system calls to the kernel (running on the main browser thread). Our standard system c

We use Web Workers as the basis for our processes, and processes perform system calls to the kernel (running on the main browser thread). Our standard system call mechanism is asynchronous and uses the standard postMessage API to/from Web Workers. When executing a system call, the language runtime saves the current call stack, and when the Web Worker gets a response to the system call, it resumes that thread. So we build these low-level primitives (kernel, system calls, processes) on top of APIs the browser provides, and extend to-JS compilers + runtimes to use these primitives.