I cannot seem to find a way to tell TypeScript with JSDoc that a
member function of some object/interface takes a type parameter.

Here is a minimal example illustrating the problem:

```js
/**
* @template {unknown} T
* @callback IGet
* @param {string} key
* @returns {T | undefined}
*/

/**
* @typedef IGetter
* @type {object}
* _@template {unknown} T // how to write this?
* @property {IGet<T>} get
*/
```

Error: Cannot find name 'T'.

That of course can be done in TypeScript, but how to do it with
JSDoc? Is it at all possible?

Link to TypeScript playground:
<https://www.typescriptlang.org/play?strictNullChecks=true&noUncheckedIndexedAccess=true&allowUnusedLabels=true&noUnusedLocals=true&noUnusedParameters=true&removeComments=true&target=1&module=1&pretty=true&isolatedModules=true&useUnknownInCatchVariables=true&exactOptionalPropertyTypes=true&noFallthroughCasesInSwitch=true&noImplicitOverride=true&noPropertyAccessFromIndexSignature=true&keyofStringsOnly=true&filetype=js#code/PQKhCgAIUgBAXApgWwA4BsCGTIG8CuAdgNaED2A7oQL6QAqUMsAxpuugEabPGQCSAcUTxGcVJgBOmZHgDO8CQEtCAc1rFEAT1GwJw-BMKy8dSAB9IRACaIAZssRXqjYOHCgI0OPE2pHd-iF4JAkdHz88Mg4AK0RmeGdIJKSYAH0EFAxsRDwiUkoaeiTgYEgAC0pIeDJICiUceDLFWQB+HVQJMj8JHzxBYQAeOgA+WhVhF3AgA>

Julio

On 25/10/2023 19:03, Julio Di Egidio wrote:
> I cannot seem to find a way to tell TypeScript with JSDoc that a
> member function of some object/interface takes a type parameter.
>
> Here is a minimal example illustrating the problem:
>
> ```js
> /**
>  * @template {unknown} T
>  * @callback IGet
>  * @param {string} key
>  * @returns {T | undefined}
>  */
>
> /**
>  * @typedef IGetter
>  * @type {object}
>      * _@template {unknown} T  // how to write this?
>  * @property {IGet<T>} get
>  */
> ```
>
> Error: Cannot find name 'T'.
>
> That of course can be done in TypeScript, but how to do it with
> JSDoc?  Is it at all possible?
>
> Link to TypeScript playground:
> <https://www.typescriptlang.org/play?strictNullChecks=true&noUncheckedIndexedAccess=true&allowUnusedLabels=true&noUnusedLocals=true&noUnusedParameters=true&removeComments=true&target=1&module=1&pretty=true&isolatedModules=true&useUnknownInCatchVariables=true&exactOptionalPropertyTypes=true&noFallthroughCasesInSwitch=true&noImplicitOverride=true&noPropertyAccessFromIndexSignature=true&keyofStringsOnly=true&filetype=js#code/PQKhCgAIUgBAXApgWwA4BsCGTIG8CuAdgNaED2A7oQL6QAqUMsAxpuugEabPGQCSAcUTxGcVJgBOmZHgDO8CQEtCAc1rFEAT1GwJw-BMKy8dSAB9IRACaIAZssRXqjYOHCgI0OPE2pHd-iF4JAkdHz88Mg4AK0RmeGdIJKSYAH0EFAxsRDwiUkoaeiTgYEgAC0pIeDJICiUceDLFWQB+HVQJMj8JHzxBYQAeOgA+WhVhF3AgA>
>
> Julio

That's the trouble with documentation programs. Users have no control
over the program's prejudices.

John

On 25/10/2023 20:03, Julio Di Egidio wrote:
> I cannot seem to find a way to tell TypeScript with JSDoc that a
> member function of some object/interface takes a type parameter.
>
> Here is a minimal example illustrating the problem:
>
> ```js
> /**
>  * @template {unknown} T
>  * @callback IGet
>  * @param {string} key
>  * @returns {T | undefined}
>  */
>
> /**
>  * @typedef IGetter
>  * @type {object}
>      * _@template {unknown} T  // how to write this?
>  * @property {IGet<T>} get
>  */
> ```
>
> Error: Cannot find name 'T'.
>
> That of course can be done in TypeScript, but how to do it with
> [TypeScript's] JSDoc?  Is it at all possible?

I was wrong, that cannot be done in TypeScript either...

Eventually, the only way I have found is to repeat the
declaration in-line, as in:

```js
/**
* @typedef IGetter
* @type {object}
* @property {<T extends unknown>(key: string) => (T | undefined)} get
*/
```

but I lose any chance to properly document that function, which
is simply quite a disaster: and not the only one, also the return
type gets messed up (becomes just `T`), to the point that I am
compelled to do something altogether different, i.e. have a
concrete object constructor.

File under the misery of an entire industry.

Julio