How to force TS to infer a generic type argument as a string literal type?

[teal spade]

My problem is basically perfectly encapsulated in this playground

You can also read the code here:

function foo<T extends number | string = string, U extends string = string>(name: U): U {
  return name
}

const bar = foo("hello")
//    ^? const bar: "hello"

// I want `U` to still be inferred as a string literal,
// even though I just explicitly gave the first generic type argument
const baz = foo<number>("bye!")
//    ^? const baz: string

[warm stirrup]

@teal spade Here's a shortened URL of your playground link! You can remove the full link from your message.

ecvanbrussel#0

Playground Link

Preview:```ts
function foo<T extends number | string = string, U extends string = string>(name: U): U {
return name
}

const bar = foo("hello")
// ^? const bar: "hello"

// I want U to still be inferred as a string literal,
// even though I just explicitly gave the first generic type argument
...```

You can choose specific lines to embed by selecting them before copying the link.

[sand anchor]

@teal spade TS does not partially infer generic params.

If you specify one, then any unspecified params get the default value (hence what = string is doing)

If you need to infer one argument and specify another, you can split the function into two functions (one which is specified, the other which is inferred)

Something like:

foo<number>()("hello");

... but also consider whether the manually specified argument is the best way to handle that: a lot of places where people are manually passing type arguments are basicaly just type casts.

[teal spade]

@sand anchor Ah, hmm, then maybe it's not solvable, I'm not sure.

Of course I gave the simplest repro I could make, but in my actual project I have a class like so:

import { Subject } from "rxjs"

class Source<
  Input extends Event = Event,
  Output = Input,
  Type extends string = string,
> extends Subject<{ type: Type, payload: Output }> {
  type: Type

  constructor(type: Type) {
    super()
    this.type = type
  }

  // and then a whole bunch more functions
}

Long story short, whoever instantiates that Source will most likely always fill in the first generic type argument:

const btnClick$ = new Source<MouseEvent>("btnClick$")

And I would just love if I could somehow get that string given to the constructor to be kept as the literal string. It just helps when hovering over a piece of code way down the line, if the IDE shows you that the type contains the literal "btnClick$" string.

Is there any way to make that work?

Should I add a generic type to the contructor function itself maybe?