Ask HN: What is the best for writing JavaScript to the client (e.g. TypeScript)

I have used Dart and TypeScript. Both have their advantages. I haven't tried other alternatives like ClosureScript or just plain ES6 and am curious about them for productivity reasons.

"Our engineers are 2 to 3 times more productive in Dart and we are delighted we switched [from GWT]" -- Manish Gupta, VP Engineering, Google Ad Words.