TypeScript: Access nested object types and associted param types inside function body?

Question

In this example i have created 2 typed objects, toggle and counter which both conform to the Config interface, each passing their own State and Action parameter types.

My question is, how can i access these Config types and their associated State and Action params inside my createStore function body? I have no idea how to type the argument so that i don't lose that information?

I have read through the TS docs and im thinking this is something generics might help with?

Answer 1

If I've understood correctly... you can specify the types for toggle and counter like this -

type ToggleType = typeof toggle;
type CounterType = typeof counter;

And then you can inline the obj parameter for your function -

const createStore = (obj: { toggle: ToggleType, counter: CounterType  }) => {
    const { toggle, counter } = obj;
}

---

Edit after comments: Something like this may help, too, if you don't always have a toggle and a counter property (ie the properties of obj are arbitrary) -

type CreateStoreContext = {
    [key: string]: Config<any, any>
}

const createStore = (obj: CreateStoreContext) => {
    Object.keys(obj).forEach(key => {    // Key is a string
        const config = obj[key];         // config is a Config<any, any>
    })

    // ....
}

The difficulty though is that you won't know what the types are for State and Actions for each property of obj - but at least you know it conforms to a Config interface.

Answer 2

I found this article which explains how to go about extracting param values from a type.

https://itnext.io/typescript-extract-unpack-a-type-from-a-generic-baca7af14e51

This makes use of conditional types, the basic syntax is as follows:

type ExtractState<C> = C extends Config<infer State, infer Action> ? State : never;