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Again, if it helps to think of it as API documentation: a bad .d.ts file is like finding an out of date API document for an old version of the API.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.