What approaches and methods of using userland for multithreading in Node.js do you know?

Node.js is originally single-threaded, but there are several approaches to implement multithreading at the userland level (user code):

1. Worker Threads — the worker_threads module allows creating separate threads that execute JavaScript code in parallel. These are full threads that can communicate with the main thread.

const { Worker } = require('worker_threads');

const worker = new Worker(`
  const { parentPort } = require('worker_threads');
  parentPort.postMessage('Hello from worker');
`, { eval: true });

worker.on('message', msg => console.log(msg));

2. Child Processes — launching separate processes using the child_process module. These are not threads, but separate processes that can communicate via IPC.

3. Asynchronous programming and event loop — although this is not multithreading in the classical sense, asynchronous operations allow efficient use of the single-threaded event loop.

4. Libraries and wrappers — some libraries implement parallelism at the userland level, using worker threads or child processes under the hood.

Thus, for multithreading in Node.js at the userland level, most often worker_threads or child_process are used, which allows performing heavy computations or parallel tasks without blocking the main thread.