I knew it had to exist (laughing out loud), and I just found it. A JavaScript operating system. Anyone who knows me knows that I joke around about “creating a new operating system coded in JavaScript”… well, here it is: JS/UIX
JS/UIX is an UN*X-like OS for standard web-browsers, written entirely in JavaScript (no plug-ins used). It comprises a virtual machine, shell, virtual file-system, process-management, and brings its own terminal with screen- and keyboard-mapping.
You can give JS/UIX a try here: http://www.masswerk.at/jsuix/