Redux Typing based on https://medium.com/@martin_hotell/improved-redux-type-safety-with-typescript-2-8-2c11a8062575

actions.ts

type FunctionType = (...args: any[]) => any;
type ActionCreatorsMapObject = {[actionCreator: string]: FunctionType};

export type ActionsUnion<A extends ActionCreatorsMapObject> = ReturnType<A[keyof A]>

export interface Action<T extends string> {
  type: T
};

export interface ActionWithPayload<T extends string, P> extends Action<T> {
  payload: P
};

export function createAction<T extends string>(type: T): Action<T>;
export function createAction<T extends string, P>(type: T, payload: P): ActionWithPayload<T, P>;
export function createAction<T extends string, P>(type: T, payload?: P) {
  return payload === undefined ? {type} : {type, payload};
};

ContentActions.ts

import {ContentItem, ContentFolder} from './ContentModels';
import {createAction, ActionsUnion} from '../lib/actions';

export const enum ContentActionTypes {
  MoveItem = '[content] move item',
  CopyItem = '[content] copy item',
};

export const ContentActions = {
  moveItem: (items: Array<ContentItem['id']>, destination: ContentFolder['id']) =>
    createAction(ContentActionTypes.MoveItem, {items, destination}),
  copyItem: (items: Array<ContentItem['id']>, destination: ContentFolder['id']) =>
    createAction(ContentActionTypes.CopyItem, {items, destination}),
};

export type ContentActions = ActionsUnion<typeof ContentActions>;

ContentModels.ts

export interface ContentItem {
  readonly id: string;
  readonly name: string;
  readonly size: number;
  readonly revision: number;
  readonly parent?: ContentItem;
}
  
export interface ContentFile extends ContentItem {
  readonly filetype: string;
}

export interface ContentFolder extends ContentItem {
  readonly files: ContentFile[];
  readonly folders: ContentFolder[];
}

export type ContentState = {
  readonly items: {[id: string]: ContentItem},
  readonly searchItems: ContentItem[],
  readonly recentItems: ContentItem[]
};

ContentReducer.ts

import {ContentState} from './ContentModels';
import {ContentActionTypes, ContentActions} from './ContentActions';

export const InitialState: ContentState = {
  items: {},
  searchItems: [],
  recentItems: []
};

export default (state = InitialState, action: ContentActions): ContentState => {
  switch (action.type) {
    case ContentActionTypes.MoveItem: {
      return state;
    }

    case ContentActionTypes.CopyItem: {
      return state;
    }

    default: return state;
  }
}