I have the following setup, where I want to use TypeScript to make sure the user is only choosing types that are allowed in the interface.

This works fine if I call the class directly, but if I use a custom generic function to pass back same class function it does not work and gives an error.

Can anyone help me understand how to fix this?

export interface IMyInterface {
  test1: {
    test1Sub: {
      test1SubSub: string
    }
  },
  test2: {
    test2Sub: {
      test2SubSub: string
    }
  }
}

export default class MyClass<T> {
  parentFunction = <A extends keyof T>(app: A) => (
    {
      childFunction: (page: keyof T[A]) => {
        console.log(app, page)
      }
    }
  )
}

// simplified for example
export function useMyCustomHook<T> (app: keyof T) {
  const settings = new MyClass<T>();
  return settings.parentFunction(app)
}

// Works when calling class directly
const myClass = new MyClass<IMyInterface>()
const parent = myClass.parentFunction('test1')
const child = parent.childFunction('test1Sub')

// does not work when calling via hook
const myHookValue = useMyCustomHook<IMyInterface>('test1')
myHookValue.childFunction('test1') // ERROR - Argument of type 'string' is not assignable to parameter of type 'never'.

I have created a playground example of this.

TypeScript Playground

CodePudding user response：

You just need an explicit type for the app parameter:

export function useMyCustomHook<T, A extends keyof T>(app: A) {
  const settings = new MyClass<T>();
  return settings.parentFunction(app);
}

const myHookValue = useMyCustomHook<IMyInterface, "test2">("test2");
myHookValue.childFunction('test2Sub')

This is because without extending keyof T, inside the scope of useCustomHook the parameter app will always be treated as the widest type keyof T. When you use it with childFunction that creates a function signature of:

childFunction: (page: keyof T[keyof T]) => {
  console.log(app, page)
}

The type keyof T[keyof T] on MyInterface resolves to an intersection of two string literal types "test1Sub" and "test2Sub", which is always never.