I am trying to pass array of objects to a function. This function has to look at all types of values behind one of the properties of its objects. These types of values should be passed as type parameter to a returned function

What types have to be filled as question marks to achieve these results?

type A = { key: 'a' };
type B = { key: 'b' };
type C = { key: 'c' };

type Xs = (A | B | C)[];

const fn1 = (arg1: ???) => {
  return {
    f2: (arg2: ???) => null,
  };
};

// should fail
fn1([]).f2('a');
// should also fail
fn1([{ key: 'a' }]).f2('c');
// correct
fn1([{ key: 'a' }, { key: 'b' }]).f2('a');

Something like Pick<ValuesType<(A | B | C)[]>, 'key'>['key'] could be used to get "a" | "b" | "c" type. But how to infer the subset of types forming the passed array remains puzzle to me.

CodePudding user response：

Try this:

const fn1 = <K extends string>(arg1: {key: K}[]) => {
  return {
    f2: (arg2: K) => null,
  };
};

Playground Link

If you must restrict the original argument to only be of type Xs or a subtype, then try this:

const fn1 = <T extends Xs>(arg1: T) => {
  return {
    f2: (arg2: T[number]['key']) => null,
  };
};

Playground Link