Typescript: Input object has subset of keys of an interface

Question

I want to be able to specify that an input object to a function should only contains keys that are present in an existing interface (it does not need to have all the keys).

interface Group {
  name: string
}

interface Person {
  id: number
  name: string
  email: string
  age: number
}

interface AddedProperties {
  groups: Group[]
}

function createPerson<D extends object>( // <-- Should convey, D is an object with some keys of Person
  personData: D
): D & AddedProperties {
  return Object.assign({}, personData, { groups: [] })
}

interface NameAndAge {
  name: string
  age: number
}

// Valid: Should be valid
const person: NameAndAge = createPerson({ name: "bob", age: 5 })
// Valid: Should be invalid as 'personality' doesn't exist on Person
const person2 = createPerson({ name: "bob", age: 5, personality: "cheerful" })

Is this possible in typescript?

Answer 1

A simple approach is using Partial<>:

function createPerson(
  personData: Partial<Person>
): Partial<Person> & AddedProperties {
  return { ...personData, groups: [] };
}

Partial takes a type and makes all of its members optional, thus allowing you to specify any subset of properties of that type.

The downside of this typing is that the returned type has no knowledge of what you put into it:

createPerson({name: "Bob", age: 5});         // OK
createPerson({name: "Bob", gender: "male"}); // Type error
createPerson({name: "Bob"}).name;            // OK
createPerson({name: "Bob"}).age;             // OK :-(

If you want to avoid this as well, check out my other answer.