Is there a way to narrow the type of an argument based on another argument to a function in Typescript?

Question

I'm trying to create a mini DSL for some filter type operations and writing some helper methods for it.

Say I have function

const equals = (left, right) => {}

This functions needs to be typed such that the left value is a field on an object, and right is a value of the type of that object.

If I do const equals = <T> (left: keyof T, right: T[keyof T]) => {} , I get close but right is narrowed down to the types available on T rather than just the type of left .

The desired behaviour is achievable with this:

const equals = <T, F extends keyof T>(left: F, right: T[F])

But this requires two generic parameters which breaks my type inference chain for the code around this function. Ideally I want to type the second parameter based on the first parameter. Is this possible?

Thanks

Answer 1

You are probably looking for something around the following lines. The solution is to move some of the type validation logic to smart constructors.

// model
type PickPropertiesOfType<T, A> = Pick<
  A,
  {
    [K in keyof A]: A[K] extends T ? K : never;
  }[keyof A]
>;

export type Filter<A> =
  | { kind: "Equals"; field: string & keyof A; value: A[string & keyof A] }
  | { kind: "EqualsField"; field: string & keyof A; value: string & keyof A };

export const equalsOtherField = <A, K extends string & keyof PickPropertiesOfType<A[K], A>>(
  field: string & K,
  value: string & keyof PickPropertiesOfType<A[K], A>
): Filter<A> => ({
  kind: "EqualsField",
  field,
  value: value,
});

export const equals = <A, K extends string & keyof A>(field: string & K, value: A[K]): Filter<A> => ({
  kind: "Equals",
  field,
  value: value,
});

// example
type ExampleEntity = {
    author: string;
    publisher: string;
    year: number;
}

const exampleEntity: ExampleEntity = {
    author: "Foo",
    publisher: "Foobar",
    year: 2000
}

const filterArr: Filter<ExampleEntity>[] = [
  equals("author", "Bar"),
  equalsOtherField("author", "publisher"),
];