Typescript, index one interface keys using another interface

Question

I have two interfaces with identical optional keys, but different values:

interface Obj1 {
  a?: string
  b?: string
  c?: number 
}

interface Obj2 {
  a: boolean
  b: string
  c: number 
}

Obj1 is served as a function argument, the other, Obj2, is the return of that function. I want the return type to identify only the given keys on Obj1. So if Obj1 contained only a and b then Obj2 will contain only a and b as well.

I tried with the approach bellow, but I get a ts error Type 'Property' cannot be used to index type 'ValueType'

type Obj1KeysWithObj2Values<KeyType extends Obj1, ValueType extends Obj2> = {
  [Property in keyof KeyType]: ValueType[Property]
}

UPDATE: The function and its call

const myFunc = <T extends Obj1>({ a, b, c }: T) => {
  const returnObj: Partial<Obj2> = {}
  if (a) {
    returnObj.a = true
  }
  if (b) {
    returnObj.b = '1'
  }
  if (c) {
    returnObj.c = 20
  }
  return returnObj as Obj1KeysWithObj2Values<T, Obj2>
}

const resultObj = myFunc({ a: 'hello', b: 'hello' })

If you try it, then you see on resultObj you get what ever you pass to the function, as long as it is in interface Obj1, regardless of Obj2.

Answer 1

Consider using Pick<Type, Keys> which is a part of standard library:

interface Obj1 {
 a?: string
 b?: string
 c?: string
}

interface Obj2 {
 a?: boolean
 b?: boolean
 c?: boolean
}

type Result = Pick<Obj2, keyof Obj1>

First argument represents source object, second argument represents a union of keys which should be picked

In your case, you also need to make an intersection of Obj1 keys and Obj2 keys:

interface Obj1 {
  a?: string
  b?: string
  c?: number
}

interface Obj2 {
  a: boolean
  b: string
  c: number
}

const myFunc = <T extends Obj1>({ a, b, c }: T) => {
  const returnObj: Partial<Obj2> = {}
  if (a) {
    returnObj.a = true
  }
  if (b) {
    returnObj.b = '1'
  }
  if (c) {
    returnObj.c = 20
  }
  return returnObj as Pick<Obj2, keyof T & keyof Obj2>
}

const resultObj = myFunc({ a: 'hello', b: 'hello' })

Playground

Answer 2

The issue is that although you know that Obj2 will always have a superset of Obj1's keys, TypeScript doesn't know that, so it needs reassuring. :-) If you want to use those generics, you can clear that error by using a conditional type to test that the property key is also present in ValueType:

type Obj1KeysWithObj2Values<KeyType extends Obj1, ValueType extends Obj2> = {
    [Property in keyof KeyType]: Property extends keyof ValueType ? ValueType[Property] : never;
// −−−−−−−−−−−−−−−−−−−−−−−−−−−−−^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
};

Playground example