Tic Tac Toe - Node

This repository includes an implementation of a simple Tic Tac Toe game with multiplayer support provided by a Node.js websocket server. You can play the game at https://ttt.jakesgordon.com, but of course you'll need to wait for a second player to join. You can always play against yourself if you use 2 browsers or 2 separate devices.

NOTE: I may - or may not - still be hosting this game by the time you read this, so YMMV.

Usage

> npm run dev        # run both client and server
> npm run dev:client # run client only
> npm run dev:server # run server only
> npm run test       # run (server) unit tests
> npm run lint       # run eslint
> npm run build      # build production assets

Repository Structure

The project has the following dependencies:

• Vite for the development server
• Typescript as the development language
• HTML and CSS - no fancy frameworks needed :-)

    └── index.html          - page layout
    └── public              - static assets
    └── src
        └── interface.ts    - abstract types used by both client and server
        └── server.ts       - THE WEBSOCKET SERVER
        └── client.ts       - THE WEBSOCKET CLIENT
        └── client
            └── header.ts   - a webcomponent
            └── board.ts    - a webcomponent

Overview

There are 3 main components

• client.ts is the client side entry point that constructs a websocket and connects to the server. It has no real game logic and simply sends commands to the server in response to user actions and then updates the UX in response to events it receives back from the server.
• server.ts is the websocket server that does all the heavy lifting and manages a lobby of players and the game board that they are currently playing. It responds to the commands it receives from the websocket clients by updating game state and sending events back.
• interface.ts contains the enums, commands, and event types shared between the client and the server.

Interface - Enums

The client and server share some simple enums and interfaces that help define some of the key concepts in a Tic-Tac-Toe game, and can be found in the interface module:

export enum Piece {
  Dot   = "dot",
  Cross = "cross",
}

export enum Position {
  TopLeft     = "top-left",
  Top         = "top",
  TopRight    = "top-right",
  Left        = "left",
  Center      = "center",
  Right       = "right",
  BottomLeft  = "bottom-left",
  Bottom      = "bottom",
  BottomRight = "bottom-right",
}

export enum WinningLine {
  TopRow       = "top-row",
  MiddleRow    = "middle-row",
  BottomRow    = "bottom-row",
  LeftColumn   = "left-column",
  CenterColumn = "center-column",
  RightColumn  = "right-column",
  DownDiagonal = "down-diagonal",
  UpDiagonal   = "up-diagonal",
}

In addition, the PlayerState enum acts as a state machine for the player who can be in any one of the following states at any given time:

export enum PlayerState {
  Joining     = "Joining",
  WaitingGame = "WaitingGame",
  TakingTurn  = "TakingTurn",
  WaitingTurn = "WaitingTurn",
  Won         = "Won",
  Lost        = "Lost",
  Tied        = "Tied",
  Abandoned   = "Abandoned",
}

The Player interface represents the minimalist representation of a player that is sent between the client and server. They must have a state and a name, and they might have a piece if they are currently playing a game.

export interface Player {
  state: PlayerState;
  name: string;
  piece?: Piece;
}

Interface - Commands

The client and server also share some important types that enable our event driven architecture where a Command can be sent to the server and an Event can be sent back to the client. We use a pattern called a discriminated union to represent all of the possible commands and their arguments.

export enum Command {
  Join   = "Join",
  Turn   = "Turn",
  Leave  = "Leave",
  Replay = "Replay",
}

interface JoinCommand {
  type: Command.Join;
  name: string;
}

interface TurnCommand {
  type: Command.Turn;
  position: Position;
}

interface LeaveCommand {
  type: Command.Leave;
}

interface ReplayCommand {
  type: Command.Replay;
}

export type AnyCommand =
  | JoinCommand
  | TurnCommand
  | LeaveCommand
  | ReplayCommand

This pattern enables our server to easily switch based on the command and perform different actions, e.g:

execute(command: AnyCommand) {
  switch(command.type) {
  case Command.Join:   return this.join(command.name)
  case Command.Turn:   return this.turn(command.position)
  case Command.Leave:  return this.leave()
  case Command.Replay: return this.replay()
  }
}

Interface - Events

Similarly, we use the same discriminated union pattern to represent all of the possible events and their arguments.

export enum Event {
  PlayerReady       = "PlayerReady",
  GameStarted       = "GameStarted",
  PlayerTookTurn    = "PlayerTookTurn",
  OpponentTookTurn  = "OpponentTookTurn",
  OpponentAbandoned = "OpponentAbandoned",
  PlayerWon         = "PlayerWon",
  PlayerLost        = "PlayerLost",
  PlayerTied        = "PlayerTied",
  UnexpectedError   = "UnexpectedError",
}

export interface PlayerReadyEvent {
  type: Event.PlayerReady;
  player: Player;
}

export interface GameStartedEvent {
  type: Event.GameStarted;
  player: Player;
  opponent: Player;
}

export interface PlayerTookTurnEvent {
  type:     Event.PlayerTookTurn;
  player:   Player;
  opponent: Player;
  position: Position;
}

export interface OpponentTookTurnEvent {
  type:     Event.OpponentTookTurn;
  player:   Player;
  opponent: Player;
  position: Position;
}

export interface OpponentAbandonedEvent {
  type:     Event.OpponentAbandoned;
  player:   Player;
  opponent: Player;
}

export interface PlayerWonEvent {
  type: Event.PlayerWon;
  player: Player;
  opponent: Player;
  line: WinningLine;
}

export interface PlayerLostEvent {
  type: Event.PlayerLost;
  player: Player;
  opponent: Player;
  line: WinningLine;
}

export interface PlayerTiedEvent {
  type: Event.PlayerTied;
  player: Player;
  opponent: Player;
}

export interface UnexpectedErrorEvent {
  type: Event.UnexpectedError;
  error: UnexpectedError;
}

export type AnyEvent =
  | PlayerReadyEvent
  | GameStartedEvent
  | PlayerTookTurnEvent
  | OpponentTookTurnEvent
  | OpponentAbandonedEvent
  | PlayerWonEvent
  | PlayerLostEvent
  | PlayerTiedEvent
  | UnexpectedErrorEvent

Again, this pattern enables our client to easily switch based on the event in order to handle it appropriately, e.g:

handle(event: AnyEvent) {
  switch(event.type) {
  case Event.PlayerReady:       return this.onPlayerReady(event)
  case Event.GameStarted:       return this.onGameStarted(event)
  case Event.PlayerTookTurn:    return this.onPlayerTookTurn(event)
  case Event.OpponentTookTurn:  return this.onOpponentTookTurn(event)
  case Event.OpponentAbandoned: return this.onOpponentAbandoned(event)
  case Event.PlayerWon:         return this.onPlayerWon(event)
  case Event.PlayerLost:        return this.onPlayerLost(event)
  case Event.PlayerTied:        return this.onPlayerTied(event)
  case Event.UnexpectedError:   return this.onError(event.error)
  }
}

The Client

The client is fairly simple and naive. It handles events from the user by sending commands to the server, and then it responds to events sent back from the server by updating the UX for the user.

It is broken up into 2 web components, the header renders the current game information to the user, while the board renders the tic-tac-toe board itself. Both are fairly simple web components that use a <template> that can be found in the main index.html file, and CSS styling declared in client.css. The UX is simple and there is no need for shadow DOM encapsulation or any reactive frameworks. They simply update various fixed elements of the UX using traditional DOM manipulation and the strategic use of css classNames to show or hide elements.

The Server

The server does the bulk of the work. It provides a web socket server that adds a new player to the lobby when a new client connects and starts listening for commands from that client. When a command is received it is passed to lobby.execute which updates the game state and sends events back to each player.

The server consists of 3 main modules:

• player - a small class to manage the player state
• board - a small class to manage the board state
• lobby - the main class that executes commands and updates the player and board state before sending events back to the client(s)

In addition, if the server detects a websocket close then it removes the player from the system.