In this post we will be building the ever popular Tic Tac Toe game in JavaScript. It's a relatively simple implementation and the most complex portion is the calculation of a win scenario. If you are relatively new to JavaScript, then I highly recommend Web Design with HTML, CSS, JavaScript and jQuery as a good starting point.

I normally start with zero code written, and I don't normally use 3rd party libraries for these things. So there are no 3rd party libraries or frameworks to setup. All you will need is your favorite coding editor, like VS Code.

Tic Tac Toe
Player 1
0
Player 2
0
Name: Tic Tac Toe
Language: JavaScript
Estimated Completion Time: 1 hour

The rules of the game sound simple enough.

In Tic Tac Toe, 2 players take turns adding their token (an X or a O) to a 3 x 3 (or N X N, for the more ambitious) grid until one player matches 3 (N) in a row in any direction. So let's set up a few basic variables to keep track of the game settings.

Variable Declaration

var winners = new Array();
var player1Selections = new Array();
var player2Selections = new Array();
var currentPlayer = 0;
var points1 = 0;    // player 1 points
var points2 = 0;    // player 2 points
var size = 3;

Here is a breakdown of what each of the function does in our program.

winners - This variable will be an array containing all of the possible combinations that are considered 'winning'.

player1Selections - We will use this array to store the blocks that player1/player2 have selected.

currentPlayer - This is the index of the player currently in play, starting with 0.

points1/points2 - These variables will be used to keep track of the ongoing games player scores.

size - This is the size of the playing board measure in width * height.

Next up I'll need to draw the tic tac toe board, which is an n x n table. In our case that n will be a 3. The following is a basic boilerplate for the game. The CSS for the elements can be found after the post at the bottom of the page.

<div class="game">
    <div class="game-title" contenteditable="true">Tic Tac Toe</div>
    <div class="player" contenteditable="true">
        Player 1
        <div style="font-size:30pt;" id="player1" class="selected" contenteditable="true">
        0
        </div>
    </div>
            
    <table id="game" class="tictactoe" style="float:left;width:45%;" contenteditable="true">
    </table>
            
    <div class="player" contenteditable="true">
        Player 2
        <div id="player2" style="font-size:30pt;" contenteditable="true">0</div>
    </div>
            
    <div class="clear" contenteditable="true"></div>
</div>

And the following function will be in charge of rendering the nxn grid onto the webpage.

// JavaScript
function drawBoard()
{
    var parent = document.getElementById("game");
    var counter = 1;

    for (var i = 0; i < 3; i++)
    {
        var row = document.createElement("tr");

        for(var x = 0; x < size; x++)
        {
            var col = document.createElement("td");
            col.innerHTML = counter;

            row.appendChild(col);
        }
        parent.appendChild(row);
    }
}

Every major step in the game is its own function. It makes it easier to edit, as changing certain parts doesn't interfere with the rest of the game elements. It also makes it much easier to test in the debugger when you can just call a function and see the outcome of that snippet immediately.

So now we have a game board. And there's not much else to do with that really. I'll add a few basic game elements, like the scores to the page, and that will look something like this.


Player 1
0
1 2 3
4 5 6
7 8 9
Player 2
0

After a win condition is met, the winning player will get a point added to their score and the game board will reset itself.

Let's break down what happens when a user makes a selection first however. And we also need to figure out what a winning case actually is.

What is a winning condition?

For an n x n tic tac toe game, a win condition is matching n selections in a straight line either horizontal, vertical, or diagonal. We don't have to start checking for a win until any user has selected 3 or more cells however, because you obviously can't win with just 2 or less elements placed on the board.

And here's the easy part. For a 3 x 3 grid, we can see what those selections will look like the following. 1,2,3 or 4,5,6, or 7,8,9.

So we have 3 sets of cells to compare against.

Horizontal: [1, 2, 3] [4, 5, 6] [7, 8, 9]

Vertical: [1, 4, 7] [2, 5, 8] [3, 6, 9]

Diagonal: [1, 5, 9] [3, 5, 7]

Any user having any of these at any point is the winner and gets a point. We'll add another variable to the list to keep these winning selections stored.

var winners = new Array();

function loadAnswers()
{
    winners.push([1, 2, 3]);
    winners.push([4, 5, 6]);
    winners.push([7, 8, 9]);
    winners.push([1, 4, 7]);
    winners.push([2, 5, 8]);
    winners.push([3, 6, 9]);
    winners.push([1, 5, 9]);
    winners.push([3, 5, 7]);
}

Ideally, we would want these permutations to be calculated automatically, as they only currently apply to a 3x3 grid.

We'll leave that as a challenge for you to take on.

So now we have a way to check if a player has won the game. But we'll need to keep track of which selections each players has. We don't want to have to go through the entire grid each time a player picks something to check if it's an 'X' or a 'O'. So we'll add 2 new variables to my list.

var player1Selections = new Array();
var player2Selections = new Array();

These arrays will keep track of which 'boxes' each user has selected.

Up next we will add an event handler to each cell click, alternating players after each one, until a winner is found, or until we run out of cells to click on. Here's a new updated version of the drawBoard() function from above.

function drawBoard() {
    var Parent = document.getElementById("game");
    var counter = 1;
    
    while (Parent.hasChildNodes()) {
        Parent.removeChild(Parent.firstChild);
    }

    for (s = 0; s < 3; s++) {
        var row = document.createElement("tr");
        
        for (r = 0; r < 3; r++) {
            var col = document.createElement("td");
            col.id = counter;
            col.innerHTML = counter;

            var handler = function(e) {
                if (currentPlayer == 0) {
                    this.innerHTML = "X";
                    player1Selections.push(parseInt(this.id));
                    player1Selections.sort(function(a, b) { return a - b });
                }

                else {
                    this.innerHTML = "O";
                    player2Selections.push(parseInt(this.id));
                    player2Selections.sort(function(a, b) { return a - b });
                }

                if (checkWinner())
                {
                    if(currentPlayer == 0)
                        points1++;
                    else
                        points2++;

                    document.getElementById("player1").innerHTML = points1;
                    document.getElementById("player2").innerHTML = points2;

                    reset();
                    drawBoard();
                }

                else
                {
                    if (currentPlayer == 0)
                        currentPlayer = 1;
                    else
                        currentPlayer = 0;
                    this.removeEventListener('click', arguments.callee);
                }
            };

            col.addEventListener('click', handler);

            row.appendChild(col);
            counter++;
        }

        Parent.appendChild(row);
    }

    loadAnswers();
}

The main additions to the function are the ability to store the players selections, adding the event handlers and checking for a winning condition. If a winning condition is found, the points get updated and then we reset the game variables and redraw the board.

This is why it's useful to break down the steps into as many functions as possible. Because drawBoard does one thing, it doesn't interfere with anything else in the game. If no win condition is found, we switch the current active player and removed the click event so that no player could click on it again. Here is the function that checks for a winning combination. It just checks the players array, with the list of predefined winners. When both having matching arrays, we have a win.

function checkWinner() {
    // check if current player has a winning hand
    // only start checking when player x has size number of selections
    var win = false;
    var playerSelections = new Array();

    if (currentPlayer == 0)
        playerSelections = player1Selections;
    else
	playerSelections = player2Selections;
    
    if (playerSelections.length >= size) {
        // check if any 'winners' are also in your selections
        
        for (i = 0; i < winners.length; i++) {
            var sets = winners[i];  // winning hand
            var setFound = true;
            
            for (r = 0; r < sets.length; r++) {
                // check if number is in current players hand
                // if not, break, not winner
                var found = false;
                
                // players hand
                for (s = 0; s < playerSelections.length; s++) {
                    if (sets[r] == playerSelections[s]) {
                        found = true;
                        break;
                    }
                }

                // value not found in players hand
                // not a valid set, move on
                if (found == false) {
                    setFound = false;
                    break;
                }
            }

            if (setFound == true) {
                win = true;
                break;
            }
        }
    }

    return win;
} 

The way checkWinner() works is that we are going to be comparing the users current selections, against the list of predetermined winning conditions that we defined in the winners array.

Detecting a Draw

Last on the list is detecting if we have a draw. That is, if no winning rows have been detected. And we can do that simply enough by checking the number of selections that the players have made. If we see that we have reached (n * n) selections, then we can assume that we have taken the board. We can add this check after each user plays their turn, by adding a conditional statement before control switches over to the next player.

if (checkWinner())
{
      if(currentPlayer == 0)
          points1++;
      else
          points2++;

      document.getElementById("player1").innerHTML = points1;
      document.getElementById("player2").innerHTML = points2;

      reset();
      drawBoard();
 }

else if (player2Selections.length + player1Selections.length == 9)
 {
     reset();
     drawBoard();
}

Reset

And lastly, once the current game is over, we want to go ahead and reset the screen and its elements.

function reset()
{
    currentPlayer = 0;
    player1Selections = new Array();
    player2Selections = new Array();
    d('player1').classList.add('selected');
    d('player2').classList.remove('selected');
}

Here is the full source down below. There's always room for improvement, and new features can be added. If you like the code and use it somewhere online, feel free to link back to me. That's always appreciated.

Full Source

<!DOCTYPE html>
<!--[if lt IE 7]>      <html class="no-js lt-ie9 lt-ie8 lt-ie7"> <![endif]-->
<!--[if IE 7]>         <html class="no-js lt-ie9 lt-ie8"> <![endif]-->
<!--[if IE 8]>         <html class="no-js lt-ie9"> <![endif]-->
<!--[if gt IE 8]><!--> <html> <!--<![endif]-->
    <head>
        <meta charset="utf-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <title></title>
        <meta name="description" content="">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <link rel="stylesheet" href="tictactoe.css">
    </head>
    <body>
        <!--[if lt IE 7]>
            <p class="browsehappy">You are using an <strong>outdated</strong> browser. Please <a href="#">upgrade your browser</a> to improve your experience.</p>
        <![endif]-->
        <div class="game">
            <div class="game-title" contenteditable="true">Tic Tac Toe</div>
            <div class="player" contenteditable="true">
            Player 1
            <div style="font-size:30pt;" id="player1" class="selected" contenteditable="true">
            0
            </div>
            </div>
            
            <table id="game" class="tictactoe" style="float:left;width:45%;" contenteditable="true">
            </table>
            
            <div class="player" contenteditable="true">
            Player 2
            <div id="player2" style="font-size:30pt;" contenteditable="true">0</div>
            </div>
            
            <div class="clear" contenteditable="true"></div>
            </div>
        <script src="tictactoe.js" async defer></script>
    </body>
</html>
    
body{
    margin:0px;
    padding:0px;
}

.game td
{
    width:100px;
    height:100px;
    border: solid 5px black;
    text-align:center;
    font-size:40pt;
    border-radius:10px;
    box-sizing:border-box;
}

.game td:hover
{
    background-color: #fff;
    cursor:pointer;
}

.game
{
    background:#efefef;
    border:solid 10px #333;
}

.game .game-title
{
    background:#222;
    color:#fff;
    padding: 10px;
    text-align:center;
}

.game .player
{
    float:left;
    text-align:center;
    width: 25%;
}

.game .player .selected
{
    border-bottom:solid 4px #00fa9a;
}

.tictactoe-board
{
    text-align:center;
    margin:0 auto;
    width:50%;
    padding-top:20px;
}

.tictactoe-board table
{
    float:left;
    width: 50%;
}

.tictactoe-board-score1
{
    float:left;
    width:20%;
}

.tictactoe-board-score2
{
    float:left;
    width:20%;
}

.clear
{
    clear:both;
}
    
    
var winners = new Array();
var player1Selections = new Array();
var player2Selections = new Array();
var timer;
var numberOfPlayers = 2;
var currentPlayer = 0;
var move = 0;
var points1 = 0;    // player 1 points
var points2 = 0;    // player 2 points
var size = 3;

function drawBoard() {
    var Parent = document.getElementById("game");
    var counter = 1;
    
    while (Parent.hasChildNodes()) {
        Parent.removeChild(Parent.firstChild);
    }

    for (s = 0; s < 3; s++) {
        var row = document.createElement("tr");
        
        for (r = 0; r < 3; r++) {
            var col = document.createElement("td");
            col.id = counter;

            var handler = function(e) {
                if (currentPlayer == 0) {
                    this.innerHTML = "X";
                    player1Selections.push(parseInt(this.id));
                    player1Selections.sort(function(a, b) { return a - b });
                    d('player1').classList.remove('selected');
                    d('player2').classList.add('selected');
                }

                else {
                    this.innerHTML = "O";
                    player2Selections.push(parseInt(this.id));
                    player2Selections.sort(function(a, b) { return a - b });
                    d('player1').classList.add('selected');
                    d('player2').classList.remove('selected');
                }

                if (checkWinner())
                {
                    if(currentPlayer == 0)
                        points1++;
                    else
                        points2++;

                    document.getElementById("player1").innerHTML = points1;
                    document.getElementById("player2").innerHTML = points2;

                    reset();
                    drawBoard();
                }

                else if (player2Selections.length + player1Selections.length == 9)
                {
                    reset();
                    drawBoard();
                }
                else
                {
                    if (currentPlayer == 0)
                        currentPlayer = 1;
                    else
                        currentPlayer = 0;
                    this.removeEventListener('click', arguments.callee);
                }
            };

            col.addEventListener('click', handler);

            row.appendChild(col);
            counter++;
        }

        Parent.appendChild(row);
    }

    loadAnswers();
}

function d(id)
{
    var el = document.getElementById(id);
    return el;
}
function reset()
{
    currentPlayer = 0;
    player1Selections = new Array();
    player2Selections = new Array();
    d('player1').classList.add('selected');
    d('player2').classList.remove('selected');
}

function loadAnswers()
{
    winners.push([1, 2, 3]);
    winners.push([4, 5, 6]);
    winners.push([7, 8, 9]);
    winners.push([1, 4, 7]);
    winners.push([2, 5, 8]);
    winners.push([3, 6, 9]);
    winners.push([1, 5, 9]);
    winners.push([3, 5, 7]);
}

function checkWinner() {
    // check if current player has a winning hand
    // only stsrt checking when player x has size number of selections
    var win = false;
    var playerSelections = new Array();

    if (currentPlayer == 0)
        playerSelections = player1Selections;
    else
	playerSelections = player2Selections;
    
    if (playerSelections.length >= size) {
        // check if any 'winners' are also in your selections
        
        for (i = 0; i < winners.length; i++) {
            var sets = winners[i];  // winning hand
            var setFound = true;
            
            for (r = 0; r < sets.length; r++) {
                // check if number is in current players hand
                // if not, break, not winner
                var found = false;
                
                // players hand
                for (s = 0; s < playerSelections.length; s++) {
                    if (sets[r] == playerSelections[s]) {
                        found = true;
                        break;
                    }
                }

                // value not found in players hand
                // not a valid set, move on
                if (found == false) {
                    setFound = false;
                    break;
                }
            }

            if (setFound == true) {
                win = true;
                break;
            }
        }
    }

    return win;
} 

window.addEventListener('load', drawBoard);
    
Walter G. is a software engineer, startup co-founder, former CTO of several tech companies and currently teaches programming for a coding bootcamp. He has been blogging for the past 5 years and is an avid BMX rider, bio-hacker and performance enthusiast.
Buy me a coffeeBuy me a coffee
If you read this far, then I hope you enjoyed this post and found it useful! Consider adding to my daily coffee funds to continue to provide better and more helpful articles in the future!
Maybe later

Add a comment

Send me your weekly newsletter filled with awesome ideas
Post comment