TypeScript

TypeScript is a language for application-scale JavaScript, adding optional types on top of the language and compiling down to readable, standards-based JavaScript that runs in any browser, on any host, on any OS. The types are there so tools can work on large codebases, and editors, refactoring and error checking read them while nothing survives into the emitted output. Microsoft maintains it under Apache 2.0, a browser playground runs it without a project, the stable release installs with npm install -D typescript, and nightly builds ship as typescript@next.