Node.js
node.js is a cross-platform server-side runtime environment implemented mostly in JavaScript, with applications for it intended to also be programmed in JavaScript. It is used in many web backends, but can also be used to run local command-line JavaScript programs.
Its effect is to make JavaScript usable as a server-side language as well as the client-side language it was traditionally. Under node.js, a JavaScript program does not have the browser-based DOM and other objects familiar to client-side developers, but it has a set of library routines to do such things as access filesystems and make and accept network connections, as a server needs to do. Many of the guides and references to node.js start off by showing a 14-line program that operates an HTTP server that outputs "Hello World", showing how easy it is to do such things in this framework. (And that program is usually repeated several more times in the course of the guide, sometimes with minor variations that may alter the line count slightly.)
