As a web-focused software engineer, I can safely say TypeScript is the best thing that happened to my work in the last decade.
Aside from the known direct benefits of safety and self-documentation, I've found over time that having a pleasant, smooth coding experience and producing elegant code required me to think differently. I work on a project with very complicated and overloaded business logic, but nowadays my code looks very... "algebraic"? state machines within state machines, exhaustive switches everywhere...
Maintaining and modifying such code has been a joy compared to the old ways. Most of the work is just adding a new member to some union or an attribute to a type, following the red trail fixing errors and voilà.
The real genius of it is that it's really not a "type" system at all: it's a contract system. The nearest thing like it was Eiffel. The new "satisfies" feature in 4.9 makes this even more clear. Honestly there's so much space to cover here, I think it's just going to keep getting better and better.
I’m curious what you’re distinguishing here. To me a type system and a contract system are identical concepts with different descriptions.
It seems like you might be highlighting the structural typing aspects of TypeScript’s type system versus nominal or concrete types in many others, but that’s been clear for most TS usage for since well before `satisfies` so I’m not sure if my interpretation is right.
But you can check contracts at compile time too. It's quite the same thing as static typing with something like refinement types. That's because, while with contracts we can add preconditions like "the size of this array passed as parameter must be a prime number", with refinement types we can define the type of arrays whose size is a prime number, and then have this type as the function argument. (likewise, postconditions can be modeled by the return type of the function)
See for example this Rust library: https://docs.rs/contracts/latest/contracts/
It will by default check the contracts at runtime, but has an option to check them at compile time with https://github.com/facebookexperimental/MIRAI
Now, this Rust library isn't generally understood as creating another type system on top of Rust, but we could do the legwork to develop a type theory that models how it works, and show the equivalence.
Or, another example, Liquid Haskell: https://ucsd-progsys.github.io/liquidhaskell/ it implements a variant of refinement types called liquid types, which is essentially design by contract checked at compile type. In this case, the type theory is already developed. I expect Liquid Haskell to be roughly comparable to Rust's contracts checked by MIRAI.
Now, what we could perhaps say is that refinement types are so powerful that they don't feel like regular types! And, while that's true, there are type systems even more powerful: dependent types used in languages like Coq, Lean and F* to prove mathematical theorems (your type is a theorem, and your code, if it typechecks, is a proof of that theorem).
Dependent types were leveraged to create a verified TLS implementation that mathematically proves the absence of large class of bugs, miTLS https://www.mitls.org/ (they discovered a number of vulnerabilities in TLS implementations and proved that their implementation isn't vulnerable), and HACL* https://github.com/hacl-star/hacl-star a verified crypto implementation used by Firefox and Wireguard. They are part of Project Everest https://project-everest.github.io/ which aims to develop provably secure communications software.