In his blog post: http://staltz.com/all-js-libraries-should-be-authored-in-typescript.html mentions:
I am a JavaScript programmer, and I make some libraries. Recently, however, I’ve been writing TypeScript for RxJS version 5 (used in Angular 2 which is also authored in TypeScript),
and I’m in the process of rewriting Cycle.js in TypeScript.
I find the cycle.js framework very interesting and I love to see it written in TypeScript, so kudo’s for André Staltz, when he accomplishes this.