Overloaded typescript function parameter does not work with async keyword

Question

I'm working on a larger project type definition file. I have simplified the code into an isolated example:

Module definition f.d.ts

export type myFunction = ((r: string) => Promise<any>) // async def
  | ((r: string, done: (err: Error | null, b?: any) => void) => void) //callback def

export interface addFunc {
  (c: string, f: myFunction): void
}

export interface FI {
  addFunc: addFunc
}

export default function f(): FI

Module implementation f.js

function f () {
  return {
    addFunc: (c, p) => {
      this[c] = p
      return this
    }
  }
}

module.exports = f

Module utilization index.ts

import f from './f'

f().addFunc('x', (r, d) => { // Compiles as expected
  d(null)
})

f().addFunc('x', async (r) => { // Error ts(7006) Parameter 'r' implicitly has an 'any' type.
  return null
})

Can you please explain why this error is happening and how I could fix it? I believe the issue is in the type definition.

Please do not comment on the implementation itself; this is an extremely stripped down and isolated piece of a large API.

Thank you for the help!

Mu-Tsun Tsai · Accepted Answer · 2019-07-03 02:25:20Z

2

You need to change it to

f().addFunc('x', async (r: string) => {
  return null
})

Without adding : string, your function is really a (r: any) => Promise<null>, which is not assignable to the type myFunction.

The first case in your example compiles because TypeScript is able to infer the types of r and d by the fact that there're two parameters, which makes it impossible to assign to the first case in the union definition of myFunction. But when a function has only one parameter, it could be assigned to a type that has one or more parameters, so TypeScript cannot automatically infer the type of r.

edited Jul 3, 2019 at 2:25

answered Jul 3, 2019 at 2:19

Mu-Tsun Tsai

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2 Comments

E.Arrowood Over a year ago

I see, I'll try this and see how it goes. I was hoping TS could infer this based on regular function overload patterns typescriptlang.org/docs/handbook/declaration-files/…

E.Arrowood Over a year ago

Marking this is as the correct answer because it is a valid fix. Ideally TS would be able to infer based on function overload; however, I'm not going to get blocked by it right now! Thank you :)

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