Well, imagine type-first programming, where you design your system first just by writing the types, look how it works, and only when satisfied go bother with implementation.
I'm not sure TypeScript is good enough for doing that (I don't know TypeScript that well), but it's simply much better than starting with code.
I recently had to write a tool to output a Swagger file from a proprietary api testing format, and writing all the interfaces for Swagger based on the Swagger specification was hugely helpful. I’m not sure I would have been capable of completing the project without TypeScript.
Also coming from JavaScript development first, TypeScript made learning C# and templates a breeze.
What if the specification changes ? Being static is has both advantages and disadvantages. You can do the checks without running the code, but it doesn't guarantee run-time correctness. I think a better strategy is to type check the actual code via inference, maybe adding some doctype comments to help the static analyzer, then add run-time checks where things are likely to break. And you can make your API easier to use by checking the parameters, throwing helpful and descriptive errors, allowing several different types as input, so the user doesn't have to convert his/her model to your model.
The spec changing is the best part! (Or conversely, the worst part when you are 100% dynamic). You change your types to match the new spec (eg adding a case to a discriminated union) and you immediately get feedback on places in your code that are definitely wrong/incomplete now. It won’t find every change you need to make but it’s a big load off and frees up some cognitive capacity for what’s left.
TypeScript works reasonably well for this approach; it's how I tend to do things. It has some gaps when you want to do really advanced stuff, but it gives you tools like typecasting for crossing those gaps. One of the great benefits of JavaScript type systems is that since they're overlaid on a dynamic language, you can just "turn them off" as needed (when the type system fails to understand that what you're doing is safe). You can comment out type checking for a particular line, or even let your whole project successfully build while type errors remain. The type system then becomes purely a tool, and never a roadblock.
> design your system first just by writing the types,
Fine.
> look how it works,
Beutiful except it doesn't work yet because it's just types that make some shape. You could literally draw the thing with pen and paper.
> go bother with implementation.
And only then discover that something you have thought of as implementation detail is one of the main problems with your solution.
No problem. I'll just refactor some type. It's easy. Less beautiful but at least it'll work. Except it doesn't. Butchering your elegant design for hours you come to hard earned realisation that the shape you imagined that you wrote and carefully named all your types to represent actually has nothing to do with the one you need.
If you are a bad person you ship your mangled typed monstrosity that you just barely made to work with traces of your struggle fossilised for any future maintainers.
If you are a good person you toss everything to into trash (save for few hard earned implementations) and start fresh with knowledge and humility wishing you worked with TypeScript where you can start with implementation and only after you figured out the hard parts, add types that reflect your solution (finding
few additional bugs in the process and getting more confident about the thing you wrote).
> You could literally draw the thing with pen and paper.
Paper does not tell you when something you draw is wrong.
> And only then discover that something you have thought of as implementation detail is one of the main problems with your solution.
What is no different from learning that something you implemented is wrong, except in that you spent less time to get the problem. Or do you expect some magical designing tool that will lead you to the right solution before you understand your problem?
> Paper does not tell you when something you draw is wrong.
With some things it does. At least better than your brain does. Typesystem does it even a bit better. But you end up with a picture not a program nonetheless, before you start writing implementation.
> What is no different from learning that something you implemented is wrong [...]
What is different is that with all the types you wrote, the process of problem discovery that happens during implementantion is stifled by what you wrote so far. Also you have additional temptation to contort your types and to leave it in a bad state because you wrote so much already and you'd very much want to avoid tossing that out.
My point is that you discover problem through experimentation and types make experimentation harder because they make the code in some ways harder to change. TypeScript avoids that by allowing you to use as much types as you want at any stage of problem/solution discovery but not forcing you to.
Well, again, what of the things you said do not apply to any form of planning?
The only benefit of a plan is that you may discover it's wrong, and it's easier to throw away than the done thing. If you are not willing to reform or throw away your plans, you are doing it wrong.
I'm not sure TypeScript is good enough for doing that (I don't know TypeScript that well), but it's simply much better than starting with code.