Making Four-In-A-Row Using JavaScript - Part 3: Making Moves

Intro

In the previous blog post, you set up your FourInARowGame class's fields.

Now you're ready to start implementing player moves and updating the state of the game accordingly.

Creating The Method

You'll start by creating a new method in the FourInARowGame class called playMove():

export default class FourInARowGame {
  // ..

  static createBoard() {
    // ..
  }

  playMove(columnIndex) {
    return {
      board: this.currentBoard,
      winner: Constants.PlayerColor.NONE,
      status: {
        value: Constants.MoveStatus.SUCCESS,
      },
      winLine: [],
    };
  }
}

Note: You're just returning mock data for now. You'll properly implement this method later on.

The playMove() method takes in a columnIndex argument. From this input, a move will be attempted and the result of the move will be evaluated and returned. The code example above shows a MoveResult object being returned in the playMove() method. A MoveResult can contain:

• The current state of the board after a move has been played
• The winner
• The status of the move
• The win line - an array of positions of 4 consecutive tokens were located if found.

Interacting With Your State Machine

Exposing Your State Machine Object

Now, to try out your code, you'll interact with your Four-In-A-Row state machine in your browser.

First, you'll need to expose the object so that the browser's console can access it. One way you can do this is by adding an instance of the FourInARowGame class to the window object. Let's do that right now. Replace the contents of src/index.js with the following:

import FourInARowGame from "./FourInARowGame.js";

window.fourInARowGame = new FourInARowGame();

Now, ensure that an HTTP server is running from the root of your project and navigate to the server's address in your web browser.

Open up the console in your browser's developer tools (The keyboard shortcut to open in it Google Chrome is CTRL + Shift + J) in your web browser then enter in: window.fourInARowGame. You should see an output of the FourInARowGame instance object. In Google Chrome it looks like this:

FourInARowGame {startingColor: 'yellow', currentTurn: 'yellow', status: 'start', board: Array(6)}

There is also the option to expand the object so you can view more details about the object.

If you don't see some sort of representation of a FourInARow instance object then please go over your code and check that you've followed the instructions correctly!

Play A (Fake) Move

Now, call the playMove() method from the FourInARow instance, in the browser console window.

First, store the state machine in an easier-to-reference variable called game:

let game = window.fourInARowGame;

Then, call the playMove() method with the columnIndex parameter set to 0:

game.playMove(0);

Your browser's console window should output something resembling the MoveResult object you discussed earlier:

{board: Array(6), winner: 'none', status: {…}, winLine: Array(0)}

When expanded, you get more details:

{board: Array(6), winner: 'none', status: {…}, winLine: Array(0)}
    board: (6) [Uint8Array(7), Uint8Array(7), Uint8Array(7), Uint8Array(7), Uint8Array(7), Uint8Array(7)]
    status: {value: 'success'}
    winLine: []
    winner: "none"
    [[Prototype]]: Object

Now that you know the type of result that you're expecting after calling playMove(), let's properly implement the method so that it returns real data based on the state of the board.

Implementing The Method (For Real This Time!)

Firstly, you'll update the playMove() method in your FourInARowGame class so that it checks the current status of the game before allowing the player to make a move.

It wouldn't make sense for a player to be able to perform a move when the game has already ended in a win or a draw.

You'll also update the current status of the game from GameStatus.START to being GameStatus.IN_PROGRESS.

playMove(columnIndex) {
    switch (this.status) {
        case Constants.GameStatus.START:
            this.status = GameStatus.IN_PROGRESS;
            break;
        case Constants.GameStatus.DRAW:
        case Constants.GameStatus.WIN:
            // The game is over at this point so
            // re-evaluate the latest board, returning the same game status
            // and board details.

            // TODO: Implement this properly
            console.log("Game already ended in win or draw. re-evaluating latest game state);
        default:
            break;
    }
}

You'll re-evaluate the latest game state when a win or draw happens later.

For now, let's continue focusing on allowing a player to make a move that changes the game's state.

You will first create a method called performMove():

performMove(columnIndex) {
  // ...
}

Now you need to create a copy of the current board that you can modify without changing the value of this.currentBoard.

To do this, you'll create a static method called deepBoardCopy():

static deepBoardCopy(oldBoard) {
  let newBoard = new Array(Constants.BoardDimensions.ROWS);

  for (let rowIndex = 0; rowIndex < Constants.BoardDimensions.ROWS; rowIndex++) {
      newBoard[rowIndex] = new Uint8Array(Constants.BoardDimensions.COLUMNS);
      for (let columnIndex = 0; columnIndex < Constants.BoardDimensions.COLUMNS; columnIndex++) {
          newBoard[rowIndex][columnIndex] = oldBoard[rowIndex][columnIndex];
      }
  }

  return newBoard;
}

Now store the board copy in a variable called nextBoard:

performMove(columnIndex) {
  let nextBoard = FourInARowGame.deepBoardCopy(this.currentBoard);
}

The next thing you need to do is to perform the move on the board, to do this you'll create a method called tryPerformMove():

tryPerformMove(columnIndex, nextBoard) {
    let isMoveValid = false;

    for (let i = nextBoard.length - 1; i > -1; i--) {
        let boardRow = nextBoard[i];
        let boardPosition = boardRow[columnIndex];

        if (boardPosition !== Constants.BoardToken.NONE) {
            continue;
        }

        boardRow[columnIndex] = FourInARowGame.playerColorToBoardToken(this.currentTurn);
        isMoveValid = true;
        break;
    }

    if (!isMoveValid) {
        return {
            status: Constants.MoveStatus.INVALID,
        };
    }

    return {
        status: Constants.MoveStatus.SUCCESS,
        board: nextBoard
    };
}

The method above checks the column in the board the player specified from the bottom to the top for an empty position and then attempts to add the current player's token to the board position. Later it returns the result.

There is a static method here called playerColorToBoardToken that was used, this sets the value in the board position to a numeric value that is associated with the current player's colour.

Add it to the FourInARowGame class too:

static createBoard() {
    // ...
}

static playerColorToBoardToken(playerColor) {
    switch (playerColor) {
        case Constants.PlayerColor.YELLOW:
            return Constants.BoardToken.YELLOW;
        case Constants.PlayerColor.RED:
            return Constants.BoardToken.RED;
        default:
            return Constants.BoardToken.NONE;
    }
}

Now you'll go back to the performMove() method and set the current game board to the board in the object returned from the tryPerformMove() method:

performMove(columnIndex) {
  let nextBoard = FourInARowGame.deepBoardCopy(this.currentBoard);

  let moveAttemptResult = this.tryPerformMove(columnIndex, nextBoard);

  if (moveAttemptResult.status === Constants.MoveStatus.INVALID) {
    return {
        board: nextBoard,
        winner: Constants.PlayerColor.NONE,
        status: {
            message: "Returned column is filled",
            value: Constants.MoveStatus.INVALID
        },
        winLine: []
    }
  }

  // From this point, the board move was successful.
  this.currentBoard = moveAttemptResult.board;
}

If the value of the moveAttemptResult.status is MoveStatus.Invalid then you return a MoveResult which has the same structure as the move data you returned in your fake playMove() method implementation.

Lastly, you need to create a method called evaluateGame() which will be used to check if the status of the game has changed and then returns a MoveResult.

For now, you'll just return the MoveResult of a successful move, which also indicates that the game is still in progress.

evaluateGame(board) {
    // From this point, you can assume that a successful move was made and the game will
    // continue on.
    return {
        board: board,
        winner: Constants.PlayerColor.NONE,
        status: {
            value: Constants.MoveStatus.SUCCESS
        },
        winLine: []
    };
}

Make sure that you return the result of this method in performMove():

performMove(columnIndex) {
  let nextBoard = FourInARowGame.deepBoardCopy(this.currentBoard);

  let moveAttemptResult = this.tryPerformMove(columnIndex, nextBoard);

  if (moveAttemptResult.status === Constants.MoveStatus.INVALID) {
    return {
        board: nextBoard,
        winner: Constants.PlayerColor.NONE,
        status: {
            message: "Returned column is filled",
            value: Constants.MoveStatus.INVALID
        },
        winLine: []
    }
  }

  // From this point, the board move was successful.
  this.currentBoard = moveAttemptResult.board;
  return this.evaluateGame(moveAttemptResult.board);
}

Now the MoveResult will show an updated board value based on the column the player placed their token in.

To complete your updates to the playMove() method, you'll add a few more lines to it:

playMove(columnIndex) {
    switch (this.status) {
        case Constants.GameStatus.START:
            this.status = Constants.GameStatus.IN_PROGRESS;
            break;
        case Constants.GameStatus.DRAW:
        case Constants.GameStatus.WIN:
            // The game is over at this point so
            // re-evaluate the latest board, returning the same game status
            // and board details.

            // TODO: Implement this properly
            console.log("Game already ended in win or draw. re-evaluating latest game state");
        default:
            break;
    }

    let moveResult = this.performMove(columnIndex);

    // Only change current turn while the game is still in progress.
    if (moveResult.status.value === Constants.MoveStatus.SUCCESS) {
        this.currentTurn = this.currentTurn === Constants.PlayerColor.YELLOW
            ? Constants.PlayerColor.RED
            : Constants.PlayerColor.YELLOW;
    }

    return moveResult;
}

You perform a move, update the current turn to the opposing player's turn then return the results of the move.

Testing Out Your Changes

You added quite a few changes, let's test them out.
Assuming you have a local HTTP server running at the root of the project, navigate to the server's address in your web browser and open the browser console window again.

First, enter the following line in your browser's console:

let game = window.fourInARowGame;

Now call the playMove() method with the columnIndex parameter set to 1 instead of 0 to add the token to the second column on the game board:

game.playMove(1);

You should see a MoveResult object returned. If you expand the object and then expand the board field, you'll see that the value 1 has been added to the bottom of the second column on the board.

If you do, congratulations!

If not, please go over this post carefully to check for mistakes you may have made.

Thanks for reading!