TypeScript: The Origin Story

I heard the origin story of TypeScript. Microsoft was beginning to build VS code and they're building it in JavaScript. They got to hundreds of thousands of lines of code or something like that. And they kind of realized that they couldn't really make forward progress anymore. Because you can't refactor JavaScript like in a large code base. It's just not possible to do it and maintain correctness without a bunch of regressions. I think 100% coverage unit tests on an app code base is even an advantage.