Is Angular 2 written in Dart or TypeScript?

Question

Dart

As far as I know Angular 2 is developed in TypeScript. But visiting the GitHub repository, I saw that 30% of the code is written in Dart and only 4% in TS. I also found such a thing as ts2dart - TypeScript to Dart transpiler. Why use a transpiler to convert TypeScript to Dart?

Answer 1

Why use a transpiler to convert TypeScript to Dart?

So it happened historically
First they wrote in Dart - cool PL, static typing, classes, ...
ECMAScript6 is only in drafts so far and xs what it will be
TS - just appeared, it is from small software, small software has bad karma
But MS invites people from open source, they they listen to the community and implement goodies
Yes, they implement it in such a way that everything-everything-all the rake that ng has collected is corrected
And then it turns out that ES6 is suspiciously similar to TS
ng2 will be done for a long time - there are opinions that 1.5 will be, but everything is delicious what will be done in ng2 backported in 1.x Well, ng2 itself will be rewritten a couple of times from 0, first to TS, and then to ES6

Answer 2

There, if you look raw, then many classes have alternative implementations on dart, ts and some incompatible with the es6 standard. But most of the code is in es6, with the js extension.
For example, here are three xhr implementations at once: https://github.com/angular/angular/tree/master/mod...