Typed vuex mapState

typed-vuex.ts

import { mapState as originalMapState } from "vuex";

// Utilities to map the record fields to their function return types
// See https://github.com/microsoft/TypeScript/issues/15763#issuecomment-364205392
type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;
type MappedOutputs<T> = {
    [P in keyof T]: () => ReturnType<T[P]>; // This is a lambda so it fulfills vuex's contract
};
type MapperRecord<S> = Record<string, (store: S) => any>;

type Expand<T> = {} & { [P in keyof T]: T[P] };

interface MapperWithNamespace<S> {
    (map: (keyof S)[]): { [K in keyof S]?: () => S[K] };
    (namespace: string, map: (keyof S)[]): { [K in keyof S]?: () => S[K] };
    <T extends MapperRecord<S>>(record: T): Expand<MappedOutputs<T>>;
}

export const mapState = originalMapState as MapperWithNamespace<iState>;