A type safe function that takes a map of state selectors and returns, - a function that takes state and returns, - a map with the same keys and the result of the selectors.

mapStateToProps.ts

/**
 * @private
 *
 * An object representing a map of selector functions that
 * accept application state and return a value.
 */
interface MapOfSelectors {
  [prop: string]: (storeState: any) => any
}

/**
 * @private
 *
 * Returns a type representing a map of values based on a map of state selectors and the values those state selectors return
 */
type MapOfPropsFromMapOfSelectors<T extends any> = {
  [P in keyof T]: T[P] extends (storeState: any) => infer B ? B : T[P]
}

/**
 * @private
 *
 * Describes a `"mapStateoProps"` function composed using a map (object) of selector functions
 */
type MSTP<T> = (storeState: any) => MapOfPropsFromMapOfSelectors<T>
/**
 * Helper function for creating `mapStateToProps` functions.
 *
 * Takes a map of selector functions, for example:
 *
 * ```typescript
 * const mapOfSelectors = {
 *   counter: (storeState: any): number => state.counter.value,
 *   user: (storeState: any): User[] => state.users
 * }
 * ```
 *
 * and returns a function that takes `state` and returns an object
 * with the same keys and the results of those selectors:
 *
 * ```typescript
 * (storeState: any) => {
 *   return {
 *     counter: 4,
 *     user: [{id: 4, username: "Bob"}],
 *   }
 * }
 */
export default <T extends MapOfSelectors>(mapOfSelectors: T): MSTP<T> => (
  storeState: any
): MapOfPropsFromMapOfSelectors<T> =>
  Object.keys(mapOfSelectors).reduce((acc, key) => {
    const thunk = mapOfSelectors[key]
    acc[key] = thunk(storeState)
    return acc
  }, Object.create(null))