What is the Node.js runtime made of?

More details in the official Node.js docs - About Node.js

#1. V8 Engine (the JavaScript engine)

• The same engine used in Google Chrome.
• It compiles JavaScript into machine code (JIT, Just-In-Time compilation).
• It is responsible for:
  • interpreting and running JS code;
  • memory management (garbage collection);
  • optimizing code at runtime.

V8 is exactly what lets Node.js run JavaScript directly, without a browser.

#2. libuv (asynchrony and I/O)

• A C library that provides:
  • the Event Loop, the main event cycle;
  • a Thread Pool for background tasks;
  • non-blocking input/output (I/O);
  • work with the filesystem, network sockets and timers.
• Thanks to libuv, Node.js can handle many requests at once without blocking.

#3. Bindings (C++ Bindings)

• A "bridge" between the JS world (V8) and native code (libuv, system libraries).
• Node.js uses Node bindings to call functions written in C/C++ from JavaScript.
• For example, when you call fs.readFile(), under the hood it reaches the operating system's syscalls through bindings.

#4. Core Modules (Node.js's built-in modules)

• Implemented on top of V8 and libuv, and include:
  • fs (filesystem);
  • http, https;
  • net, dns;
  • path, url, stream, events, and others.
• These modules are the part of the Node.js API written in C++ and JavaScript so developers can work at a high level.

#5. Event Loop

• The heart of Node.js's asynchronous model.
• An infinite loop that:
  1. pulls tasks from queues (timers, I/O, callbacks, and so on);
  2. distributes them across phases;
  3. runs them in priority order.
• The Event Loop is driven by the libuv library.

#6. Thread Pool

• Inside libuv there is a pool of 4 threads (by default).
• It runs heavy operations that don't depend on the main Event Loop thread, for example file operations, DNS, cryptography.

#7. C/C++ Core and the Node API

• Node.js's low-level parts, written in C/C++, handle:
  • initializing the environment;
  • memory management;
  • interacting with the OS;
  • supporting process, Buffer, streams, and others.

#8. The Node.js CLI and REPL

• The shell that runs your code (node script.js).
• Supports REPL (interactive command execution).
• Initializes the runtime, wiring up V8, libuv and the core modules.

#The short structure: