How can I link function parameter types in TypeScript?

Question

I have an object with values of type string and number :

interface Person {
  name: string;
  age: number;
}

I have a function that changes the object with one parameter as the field index, and the other as the field value:

const person = {name: "Fred", age: "30"};

const changePerson = (index: keyof Person, value: string | number): void => {
  person[index] = value;
}

The problem is that, as TypeScript sees it, index can be "name" and value can be a number, but that would never happen. I'm not sure how to link index and value to tell TypeScript that the types are linked in a specific way.

Can anyone help?

I was able to get rid of the errors by using a case statement but that would not be transpiled out as TypeScript normally would:

const changePerson = (index: keyof Person, value: string | number): void => {
  switch(index) {
    case "name":
      person[index] = value as string;
      break;

    default:
      person[index] = value as number;
  }
}

Answer 1

You could do it with generics: <T extends keyof Person>(index: T, value: Person[T]): void

This basically tells the type system that the second parameter has to be whatever the first one points to on the type.

The full example on Typescript playground