Typescript does not detect that property is a function

Question

I am using the intl-ts library. The following function does not compile because it complains that lang[result] is not known to be executable:

function convertResult<
  T extends Messages &
    { [P in K]: (fieldName: string, p1: P1, p2: P2, p3: P3, p4: P4) => string },
  K extends keyof T,
  P1 = any,
  P2 = any,
  P3 = any,
  P4 = any
>(
  result: K | null,
  params: [P1, P2, P3, P4],
  lang?: Intl<T>,
  fieldName?: string
): boolean | string | null {
  if (result === null) {
    return lang ? null : true
  } else {
    if (lang) {
      return lang[result](fieldName, ...params)
    } else {
      return false
    }
  }
}

I believed that the definition of K (type of result) would be enough for the compiler to know that lang[result] is indeed a method which takes the appropriate parameters.

So, I wonder if the problem is:

something I missed in this code.
something I missed in intl-ts (I am the maintainer of this package).
or something too complex for Typescript and maybe I should open an issue for it.

A simple example is provided here

Could you setup simple example in playground to reproduce the problem? — Aleksey L.
– Aleksey L., Commented Sep 21, 2018 at 8:48
The Messages & is the part that is causing problems. What does that interface contain ? — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented Sep 21, 2018 at 9:08
@TitianCernicova-Dragomir Please look at Messages source code — Stéphane Veyret
– Stéphane Veyret, Commented Sep 21, 2018 at 9:15
@AlekseyL. I edited the question to add a link to playground example. — Stéphane Veyret
– Stéphane Veyret, Commented Sep 21, 2018 at 9:23

Titian Cernicova-Dragomir · Accepted Answer · 2018-09-21 09:27:44Z

2

The problem is cause because of the intersection with Messages. Typescript will not properly infer the corect type for the index access because of it.

In this particular case I would just remove the intersection. Don't think it servers a major role, it just restates that T must contain function or strings, the mapped type you use to constrain T is much more restrictive, so if an object conforms to the constraint of T it will conform to Messgaes,

function convertResult<
    T extends { [P in K]: (fieldName: string, p1: P1, p2: P2, p3: P3, p4: P4) => string },
    K extends keyof T,
    P1 = any,
    P2 = any,
    P3 = any,
    P4 = any
    >(
        result: K | null,
        params: [P1, P2, P3, P4],
        lang?: T,
        fieldName?: string
    ): boolean | string | null {
    if (result === null) {
        return lang ? null : true
    } else {
        if (lang) {
            return lang[result](fieldName, ...params)
        } else {
            return false
        }
    }
}

answered Sep 21, 2018 at 9:27

Titian Cernicova-Dragomir

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3 Comments

Stéphane Veyret Over a year ago

This works well by both removing Messages and modifying lang to directly be of type T. Anyway, I would say it is a bug in Typescript compiler and requires an issue. Do you think the same?

Titian Cernicova-Dragomir Over a year ago

@StéphaneVeyret I think it's a bit more complicated.. there are lots of problems when you want to go from generics to concrete types inside a function .. I would have expected it work, but I expect a lot of things to work and they don't always work because of other constraints. I definitely think it's work a github issue to get an official possition

Stéphane Veyret Over a year ago

I'll open an issue then, and let the team decide if it should be corrected or not. Thank you.

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