Creating a method that can adopt the return type of the nested function?

I have a function that takes in a callback and returns the value obtained using the useSelector hook from the react-redux library.

Is there a way to utilize the return type of useSelector within my wrapper function?

import { shallowEqual, useSelector } from 'react-redux';

type SelectorFunction = (state: AppState) => /* TypeOfReturn */;

export default (selector: SelectorFunction) => useSelector(selector, shallowEqual);

Currently, my function outputs any, but I would like to avoid manual type assertions like this:

const user = useShallowEqualSelector(state => state.userAuth.user) as User | null;

If I only use useSelector directly, the type of user is correctly inferred as User | null:

const user = useSelector((state: AppState) => state.userAuth.user);

Answer №1

Here, the concept is to establish a fixed state type (AppState) while allowing the return type of the selector function to be flexible, depending on the specific selector function provided as a parameter.

To achieve this, you can utilize a parametrized type:

export default <TReturn>(selector: (state: AppState) => TReturn) => useSelector<AppState, TReturn>(selector, shallowEqual)

By using this approach, TypeScript can automatically deduce the return type for useSelector based on the argument passed as the selector function-parameter. This differs from simply "utilizing the return type of useSelector", as useSelector itself features a parametrized return type, while we are instead assigning the return type to the higher-order function.

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