JavaScript example:

https://www.typescriptlang.org/play/?filetype=js#code/PTAEAE...

Almost equivalent typescript code:

https://www.typescriptlang.org/play/?#code/C4TwDgpgBA6glsAFg...

(I had to make it a little bit different from the JS code to make it compile)

(Well, this is not exactly about arrow function I guess. I remembered that part wrong.)

Note that I cannot make the type check in JS code to pass. Whatever I do, there is always a error. Meanwhile, it does not take much to TS code to work.

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culi a day ago | parent [-]

Try this out. I think it's a much more faithful representation of your TypeScript example too. JSDoc just wants you to be explicit about the default for generic slots (which is `unknown` in TypeScript).

https://www.typescriptlang.org/play/?filetype=js#code/PTAEAE...

Hover over the variables and you should see that the type inference is working just the same as in your TypeScript example

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g947o a day ago | parent [-]

Ah I see. Looks like you avoided using the "@callback" tag but instead used the "@typedef" tag directly. Thanks!

I do think it illustrates a problem with TypeScript's support for JSDoc though. You see, I started with the code in JS and could not make it work, after which I translated it to TS. In JS/JSdoc, "@callback" is the "idiomatic" way of defining a function callback type with JSDoc. (It also makes it easier to add documentation for each parameter if necessary.) And indeed, @callback works the most of the time, except in such cases where these JSDoc tags don't work nicely together, and these alternatives become necessary.

	▲	culi a day ago \| parent [-]
		This is absolutely a fair point. JSDoc predates TypeScript by over a decade. It's not until more recently that the syntaxes have started to converge. My brain definitely works in TypeScript so I tend to translate from there. I definitely consider myself more familiar with TypeScript than with JSDoc, but sometimes (e.g. here) that's a benefit not a weakness