jest.config.js

module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
};

package.json

{
  "name": "stas",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "jest"
  },
  "keywords": [],
  "author": "Joel Shinness <[email protected]> (http://joelshinness.com)",
  "license": "ISC",
  "dependencies": {
    "@types/jest": "^24.0.17",
    "@types/react": "^16.8.24",
    "@types/react-dom": "^16.8.5",
    "jest": "^24.8.0",
    "parcel-bundler": "^1.12.3",
    "react": "^16.8.6",
    "react-dom": "^16.8.6",
    "ts-jest": "^24.0.2"
  },
  "devDependencies": {
    "typescript": "^3.5.3"
  }
}

reducer.test.ts

import { TicTacToeState, ticTacToeReducer, initialState } from "./reducer";

describe('TicTacToeReducer', () => {
  it('returns the initial state if the state is undefined', () => {
    const initial:TicTacToeState = ticTacToeReducer(undefined, null);
    expect(initial).toEqual(initialState);
  });
  it('sets a spot', () => {
    const afterMove = ticTacToeReducer(initialState, {type: 'move', spot: 4});
  
    
    expect(afterMove).toEqual({
      ...initialState,
      grid: ['empty', 'empty', 'empty', 'empty', 'X', 'empty', 'empty', 'empty', 'empty' ],
      currentPlayer: "O"
    } as TicTacToeState)
  })
});

reducer.ts

type Reducer<TState, TAction> = (state:TState | undefined, action:TAction) => TState;

export type PlayerMark = "X" | "O";
export type CellMark = PlayerMark| "empty"
export interface TicTacToeState {
  grid: CellMark[];
  currentPlayer: PlayerMark;
};

export const initialState:TicTacToeState = {
  grid: [...new Array(9).fill("empty")].map(():CellMark => "empty"),
  currentPlayer: "X"
};

interface MoveAction { type : 'move', spot: number };
interface ResetAction { type: 'reset'};
interface SomeThirdAction{ type: 'third thing'};
interface BlankAction {type: 'blank'}
type TicTacToeAction = MoveAction | ResetAction | SomeThirdAction | BlankAction;

export function _ticTacToeReducer(state:TicTacToeState | undefined, action:TicTacToeAction | null):TicTacToeState {
  if(state === undefined) return initialState;
  if(action === null) return state;
  switch(action.type){
    case "move": {
      action
      const newGrid = [...state.grid];
      newGrid[action.spot] = state.currentPlayer;
      return {
        ...state,
        grid: newGrid,
        currentPlayer: state.currentPlayer === "X" ? "O": "X"
      }
    }
  }
  return state;
}

export const ticTacToeReducer: Reducer<TicTacToeState, TicTacToeAction | null> = _ticTacToeReducer


// let reducerA: Reducer<AState, AppAction>;
// let reducerB: Reducer<BState, AppAction>;

// type CombinedStates = {
//   theAs: AState,
//   theBs: BState
// };


// let combinedReducer: Reducer<CombinedStates, AppAction> = combineReducer({
//   theAs: reducerA,
//   theBs: reducerB
// })

tsconfig.json

{
  "compilerOptions": {
    "strictNullChecks": true,
    "target": "es6"
  }
}