[rikroots]

> Once in a while I come across an obscure library without TS typings, in which case I would generally provide a `.d.ts` file for it, starting with a catch-all `any` type for its exports, later potentially specifying it further if useful and feasible. This is by far the least fun part.

If you have time, I have a question. Note that my professional exposure to Typescript projects is minimal.

I've built a Javascript library which lives in GitHub, NPM, etc (I won't spam the link). It is entirely module-based Javascript.

My understanding is that while Typescript-based projects can use non-TS JS libraries (because TS is a superset of JS), using that library would require more work - initial code, and maintenance - than using an equivalent library either written in TS, or one that includes a TS Declaration File.

If yes to the above, then - given that I'm serious about my library and want other people to use it if it meets their project needs - I assume I need to write a .d.ts Declaration File (or maybe several?) for it.

- How much initial work are we talking here: weeks? Months?

- After the initial work, how much additional work is required to maintain the .d.ts files(s)?

- How much damage could a poorly written .d.ts file do to a library?

- I've seen the Microsoft documentation[1] and ... this looks like a massive amount of work for my library. I know how popular and useful TS has become but, at the end of the day, would I be wasting my time doing what would be, for me, unpaid work?

[1] https://www.typescriptlang.org/docs/handbook/declaration-fil...

[status_quo69]

Sort of... Your maintenance burden just becomes higher as a consumer of the library because you can't rely on the compiler to tell you if something goes wrong. But if you have a good testing harness you'll probably figure that out eventually anyways.

To generate typings for your library, you can start with converting from jsdoc if you have docstrings with types, or if you want to infer types at runtime during a testing suite you can use something like https://github.com/microsoft/dts-gen

I've never used dts Gen but it looks promising as a starting point. But as an additional point, only the public functions and classes really need type declarations, so unless you export all of your functions that could cut down on the burden to write up the types.

[WorldMaker]

I've written a lot of .d.ts files, hopefully I can help.

> My understanding is that while Typescript-based projects can use non-TS JS libraries (because TS is a superset of JS), using that library would require more work - initial code, and maintenance - than using an equivalent library either written in TS, or one that includes a TS Declaration File.

More work, yes, but often not a lot of it depending on need. (You can just tell Typescript some modules are `any` and treat them as a black box.)

It's not so much "more work" as it is "less confidence". If you are looking for trust from TS developers in your library, a TS Declaration is API documentation directly in their IDE that does basic sanity/usage checks for them.

> How much initial work are we talking here: weeks? Months?

I find it is usually in the order of hours. The variables are the size of your library and in particular its external API surface and how well your API is already documented. A .d.ts file is just another sort of API documentation. If you've got good JSDOC comments, for instance, that does a lot of the work already. In some cases you can get a .d.ts file for "free" from Typescript if you follow a couple patterns in your JSDOC comments and run Typescript on your JS files. (It might also give you errors where your code doesn't agree with type hints in your JSDOC comments, which you may or may not find useful to discover potential bugs or edge cases in your code.)

> After the initial work, how much additional work is required to maintain the .d.ts files(s)?

It depends on your API changes, especially breaking changes. I've told a number of projects to feel free to @ me when they make API changes and I'll PR a .d.ts update and it's rarely more than a couple minutes to look at what their breaking API changes are and what update what needs to be updated.

> How much damage could a poorly written .d.ts file do to a library?

Again, it's mostly a "loss of confidence" style issue. A poorly written .d.ts isn't going to stop a Typescript user from using your library, it's mostly at worst just going to get in their way and they have to use an escape valve (such as a cast to `any` telling Typescript to get out of the way) and it is no worse than having no .d.ts.

Again, if it helps to think of it as API documentation: a bad .d.ts file is like finding an out of date API document for an old version of the API. It can still offer sign posts to what the expected API is supposed to be, it just requires the user of the library to distrust that documentation and either read the source directly or find other documentation sources (StackOverflow or example code from other users, for example).

> I've seen the Microsoft documentation[1] and ... this looks like a massive amount of work for my library. I know how popular and useful TS has become but, at the end of the day, would I be wasting my time doing what would be, for me, unpaid work?

As with any open source work it is a trade-off and you need to make that determination based on what you are comfortable with maintaining. That's why whenever I've written .d.ts files for libraries I don't maintain, I make sure to explain the maintenance effort, often volunteer to try to do my best to keep it maintained, and also realize and respect that not every library wants to maintain their own .d.ts.

It's also why you don't necessarily need to build the types yourself if you aren't (yet) comfortable learning Typescript or switching to Typescript. (Aside: I find that curiosity in adding your own .d.ts file is often a baby step for learning where Typescript helps you in making a better library, and some of the projects I first helped maintained a .d.ts eventually learned and switched to Typescript themselves.)

If you can find a user of your library that uses Typescript, they may already have a partial type definition you can start from and/or be willing to help you write/maintain the type definitions.

There is a central repository called DefinitelyTyped that happily maintains shared type definitions for a large number of libraries that don't intend to or cannot own and/or maintain their own type definitions. It publishes to an @types/ organization on npm. Your library may already be on there if you have enough users. In that case you also already have a path to starting with something someone else wrote, and GitHub username's to contact and reach out for additional help. If you aren't already in DefinitelyTyped and don't know specific users of your library that use Typescript who could help contribute, there are Definitions Wanted tags in DT's Issue tracker on GitHub and sometimes you can find help from DT's large number of contributors.

Hope that helps.

[raihansaputra]

On a tangent, but do you have any recommended readings specifically on writing .d.ts? I don't have a lot of experience on typed languages and I'm not too comfortable writing them, but I realize the value of having them on my project.

[WorldMaker]

I would suggest starting on the experience with typed languages in general first. Starting with the beginning of the Typescript handbook and trying working with things for instance directly in the Typescript Playground. You are going to want all of the basics when writing a .d.ts, and sometimes you aren't going to need much more than the basics.

The other thing I'd recommend is if your focus really is just specifically on writing .d.ts files, and you aren't expecting to use more Typescript directly yourself: get more familiar with and start regularly using JSDoc comments. Typescript in JS mode already understands and can do a lot with JSDoc comments. As I mentioned, it will sometimes give you a fully formed .d.ts if you ask and you have enough JSDoc comments, you'll even get the advantage of some of its type analysis in your project (not as much as if you were writing Typescript directly, but more than just a linter will do). If you use an editor such as VSCode where the JS experience is already backed by the Typescript language service you'd start to see some things almost Typescript-like light up in your development experience as you add more JSDoc comments (maybe even to help explain why you might want to use more Typescript directly).

https://en.wikipedia.org/wiki/JSDoc

[raihansaputra]

My focus is not specifically on just writing .d.ts, but mostly being capable enough to adapt JS libraries and type them for my needs. Thanks for the recommendation, will muck around with TS for now.

[rikroots]

> Hope that helps.

Thank you! This is a fantastic response (now bookmarked).

I think it best not to do the work now - the library is still evolving and keeping the basic documentation up-to-date (inline comments, but not in JSDOC format) is already an overhead. But this is something I'll need to address at some point. I'll add it to the library's roadmap.