Typescript type that is method of a module

Question

How do I declare a type that is a method of an imported module?

Suppose I have a simple file with functions:

// MyModule.ts

export const Method1 = () => {}
export const Method2 = () => {}
export const Method3 = () => {}

Now suppose I wanted to make a cool function that took one of those functions above as a parameter and then called it.

// OtherModule.ts

import * as methods from './MyModule'

//pesudocode that doesn't work
const CoolFunction = (method: typeof methods) => {
    method()
}

I want the method I pass to CoolFunction to be type checked. How do I do that?

// LastModule.ts
import * as m from './MyModule'
import { CoolFunction } from './OtherModule'

// typescript should yell at me if I try to do this
const badFunction = () => void
CoolFunction(badFunction)

// but any of these should be allowed
CoolFunction(m.Method1)
CoolFunction(m.Method2)
CoolFunction(m.Method3)

So just one of those 3 functions and no others ?

Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir

2021-12-17 21:00:07 +00:00
Commented Dec 17, 2021 at 21:00 — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented Dec 17, 2021 at 21:00
@TitianCernicova-Dragomir updated

LCIII
– LCIII

2021-12-17 21:08:01 +00:00
Commented Dec 17, 2021 at 21:08 — LCIII
– LCIII, Commented Dec 17, 2021 at 21:08

Titian Cernicova-Dragomir · Accepted Answer · 2021-12-17 21:06:25Z

3

You could use branded type on methods.

// Should work with imports too
namespace methods {
    const methodKey = Symbol.for("methodKey");
    export const Method1 = Object.assign(() => {}, { __brand: methodKey });
    export const Method2 = Object.assign(() => {}, { __brand: methodKey });
    export const Method3 = Object.assign(() => {}, { __brand: methodKey });
}

const CoolFunction = (method: typeof methods[keyof typeof methods]) => {
    method()
}


const badFunction = () => { }
CoolFunction(badFunction) //error

CoolFunction(methods.Method1) // ok

Playground Link

answered Dec 17, 2021 at 21:06

Titian Cernicova-Dragomir

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1 Comment

Aluan Haddad Over a year ago

I like this, but wouldn't it be better to use a unique symbol which can't be retrieved and can't clash in the registry?

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