I've found that there's almost no "first class" support for TypeScript in similar apps like JSFiddle, Codepen, Plunker, etc. If they do support it, it's only the syntax, not the autocomplete and error highlighting (the cool parts!). So I decided to try making my own.
Here are a few quick things I've written with it so far: https://goo.gl/zZy697 ("Flocking" behavior in a simple particle system) https://goo.gl/1q2kmr (Draw tiling patterns on a canvas) https://goo.gl/aio1rm (CSS-only card flip animation)