In the earlier versions of TypeScript, I often stuck with something natural in JavaScript but cannot express the same thing with type annotations - It's like opening a pandora box, once some advanced type operators have been introduced, whenever I played with them I would quickly found more type operators to be desired.

And so it did. Things were getting better over years, there were a lot more type operators, mapped types, the infer keyword, etc. I feel it points to a direction: There is still a lot of existing JavaScript functions' type depend on the input which cannot be well-typed. For example, there are functions similar printf in most languages, or return type T when 0 is passed otherwise return type U. To satisfy those cases, we'll basically reinvent a whole new programming language in the type system - if the new language happens to be JavaScript itself it would be similar to other dependent type systems.