Typescript ReturnType issue

Question

Im trying to get the correct return type based on the method key of an interface. Typescript is showing an issue here but i cant work out why.

interface Provider {
  reboot?(a: string): (string | null)[]
  cc?(): Promise<string>
}
type ProviderReturn<K extends keyof Provider> = ReturnType<Provider[K]>

Playground

Answer 1

Your interface defines an object with 2 potentially undefined methods. ReturnType does not work on undefined

You can do follwing with Nonnullable

interface Provider {
  reboot?(a: string): (string | null)[]
  cc?(): Promise<string>
}
type ProviderReturn<K extends keyof Provider> = ReturnType<NonNullable<Provider[K]>>

If the optional part matters, you can use a conditional type:

interface Provider {
  reboot?(a: string): (string | null)[]
  cc?(): Promise<string>
  foo(): string
}

type ProviderReturn<K extends keyof Provider> = undefined extends Provider[K] ? ReturnType<NonNullable<Provider[K]>> | undefined : ReturnType<NonNullable<Provider[K]>>

type a = ProviderReturn<'reboot'> // (string | null)[] | null; 
type b = ProviderReturn<'foo'> // string