How to construct type for "Object having path P"?

[cjol]

🤔 Question

I'm trying to write a curried "select" function which plucks a value from inside a given object. The type for the non-curried version looks like this:

declare const selectPlain: <T extends object, P extends Array<A.Key>>(
  item: T,
  ...path: P
) => O.Path<T, P>;
const strVal = selectPlain({ a: { b: '' } }, 'a', 'b');

For the curried version, the best I've come up with is this:

declare const selectCurried: <P extends Array<A.Key>>(
  ...path: P
) => <T extends object>(item: T) => O.Path<T, P>;
const selectFooBar = selectCurried('foo', 'bar');

What I don't like about this is the T extends object generic. Because of this selectFooBar will appear to accept any object. I really want to end up with

const selectFooBar : <T extends {foo: {bar: any}}>(item: T) => O.Path<T, P>;

To do that I think I need to construct the type {foo: {bar: any}} from ["foo", "bar"]. Is that possible?

Search tags, topics

#typescript #path #get #select #curry

[millsp]

Hey @cjol, yes this is very easy to achieve. We have Object.P modules that allow operating on objects at a Path. So you are looking for Object.P.Record https://millsp.github.io/ts-toolbelt/modules/_object_p_record_.html

[cjol]

Thanks, I was sure there would be a solution! For completeness, here is my final solution:

declare const selectCurried: <P extends Array<A.Key>>(
  ...path: P
) => <T extends O.P.Record<P, any>>(item: T) => O.Path<T, P>;
const selectFooBar = selectCurried('foo', 'bar');
const str = selectFooBar({ foo: { bar: 'hello' } });
// @ts-expect-error
const innerVal = selectFooBar({});

[millsp]

Glad I could help, have a nice day.