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I have been using TypeScript in JS[1] mostly seamlessly. Types are annotated using doc comments above the thing your annotating. It is a bit hard to define com

I have been using TypeScript in JS[1] mostly seamlessly. Types are annotated using doc comments above the thing your annotating. It is a bit hard to define complex types. E.g. I couldn’t for the life of me annotate the return value of the pipe function: function pipe(init, ...ops) { return ops.reduce((acc, op) => op(acc), init) } For these instances they suggest writing a declaration file[2]. However I have found the documentation isn’t really clear on this usecase (in fact I find the documentation of TypeScript somewhat lacking in general). But overall getting these typehints into my editor environment has been worth it. And I haven’t had to sacrifice the save → run workflow. 1: https://www.typescriptlang.org/docs/handbook/type-checking-javascript-files.html https://www.typescriptlang.org/docs/handbook/type-checking-j... 2: https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html https://www.typescriptlang.org/docs/handbook/declaration-fil... EDIT: An example annotated JS file for typescript: https://github.com/runarberg/strawberry-router/blob/master/src/elements/router-view.js https://github.com/runarberg/strawberry-router/blob/master/s... `npm run check` just runs typescript’s type checker, there is no compile step.

bgschiller 6y ago
> E.g. I couldn’t for the life of me annotate the return value of the pipe function Don't feel bad, no one can do that (yet! I think it should be possible with Variadic Tuple Types, coming in TS 4). There are a couple of attempts[1][2], but they all lack in some way. Redux does it with a boatload of overloads [3]. 1: https://dev.to/ascorbic/creating-a-typed-compose-function-in-typescript-3-351i https://dev.to/ascorbic/creating-a-typed-compose-function-in... 2: https://stackoverflow.com/a/53066700/1586229 https://stackoverflow.com/a/53066700/1586229 3: https://github.com/reduxjs/redux/blob/686d29b5d4e2dcb6709c16e31b79ecde90763156/index.d.ts#L451-L541 https://github.com/reduxjs/redux/blob/686d29b5d4e2dcb6709c16...
xsmasher 6y ago
If I understand your pipe function correctly - it takes an init value and a series of functions that map a type T to the same type T - then this should do it. function pipe<T>(init: T, ...ops: ((x: T) => T)[]): T{ let r:T = ops.reduce((acc:T, op) => op(acc), init); return r; } I had 5 years experience fixing type errors in C++; typescript seems like a dream in comparison. Edit: I just realized your operations probably DON'T return the same type that they are passed; they probably work in a chain like (t:T) => U, then (u:U) => V, etc. Nevermind!
- runarberg 6y ago
  Yup, the annotation should be something like: type Ops<T, R, N> = [ (x: T) => N[0], ...<n>((x: N[n]) => N[n + 1]), (x: N.last) => R, ]; function pipe<T>(init: T): T; function pipe<R, T, Ops>(init: T, ...ops: ...Ops<T, R>): R;

bgschiller 6y ago

> E.g. I couldn’t for the life of me annotate the return value of the pipe function Don't feel bad, no one can do that (yet! I think it should be possible with Variadic Tuple Types, coming in TS 4). There are a couple of attempts[1][2], but they all lack in some way. Redux does it with a boatload of overloads [3]. 1: https://dev.to/ascorbic/creating-a-typed-compose-function-in-typescript-3-351i https://dev.to/ascorbic/creating-a-typed-compose-function-in... 2: https://stackoverflow.com/a/53066700/1586229 https://stackoverflow.com/a/53066700/1586229 3: https://github.com/reduxjs/redux/blob/686d29b5d4e2dcb6709c16e31b79ecde90763156/index.d.ts#L451-L541 https://github.com/reduxjs/redux/blob/686d29b5d4e2dcb6709c16...

xsmasher 6y ago

If I understand your pipe function correctly - it takes an init value and a series of functions that map a type T to the same type T - then this should do it. function pipe<T>(init: T, ...ops: ((x: T) => T)[]): T{ let r:T = ops.reduce((acc:T, op) => op(acc), init); return r; } I had 5 years experience fixing type errors in C++; typescript seems like a dream in comparison. Edit: I just realized your operations probably DON'T return the same type that they are passed; they probably work in a chain like (t:T) => U, then (u:U) => V, etc. Nevermind!

runarberg 6y ago
Yup, the annotation should be something like: type Ops<T, R, N> = [ (x: T) => N[0], ...<n>((x: N[n]) => N[n + 1]), (x: N.last) => R, ]; function pipe<T>(init: T): T; function pipe<R, T, Ops>(init: T, ...ops: ...Ops<T, R>): R;