Node.js Fundamentals

Node.js is an open-source, cross-platform JavaScript runtime. It embeds the V8 engine and adds APIs for files, processes, networking, timers, and other operating-system work. A Node.js program runs as an operating-system process, not inside a browser page.

That boundary matters. JavaScript supplies the language: values, functions, objects, promises, and module syntax. Node.js supplies the host environment. Browser objects such as document are absent unless a library creates an equivalent. Node.js instead exposes built-in modules such as node:fs, node:http, and node:stream.

Runtime architecture

A typical Node.js application has four cooperating layers:

Your JavaScript defines application behavior and callback functions.
V8 parses and executes JavaScript.
Node.js bindings and built-in modules expose host capabilities to JavaScript.
libuv and the operating system coordinate asynchronous I/O, an event loop, and a worker pool for selected operations.

The main JavaScript thread runs one callback at a time. Node.js can still keep many I/O operations in progress because the operating system and libuv perform or monitor work outside that callback. When an operation can continue, its callback becomes eligible to run on the event loop.

This design is concurrency, not automatic parallel JavaScript. A long calculation inside one callback prevents the event loop from starting another callback. Selected APIs use the libuv worker pool, and CPU-bound JavaScript can use worker threads, but ordinary callbacks do not receive a thread each.

Node.js Fundamentals

Runtime architecture

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