Node.js is a JavaScript runtime environment, a program that runs JavaScript code outside a web browser, on a server or a developer's computer. It runs the V8 JavaScript engine, the engine at the core of Google Chrome. The site names the OpenJS Foundation and the Node.js contributors as its copyright holders.
Developers use it to write servers, web apps, command line tools and scripts. It is asynchronous and event-driven: a Node.js app runs in a single process, and when it reads from the network, a database or the file system, it does not wait. It continues with other work and runs a callback (a function given in advance) when the response comes back. In this way one server can handle thousands of connections at the same time without one thread per request. The standard library includes modules for HTTP servers, files, cryptography and a test runner, and npm, the package manager, installs libraries written by others.
Node.js is free, open source and runs on several operating systems.