Creating a Simple JavaScript Game Board

Over at the Web Cash blog they have posted two tutorials on Game development in JavaScript, Ajax and PHP. The first tutorial explains the concept and describes how to make a game board.

Below is an excerpt from the first tutorial.

Game Development with JS, AJAX, and PHP

How can we use JS and PHP - connected through AJAX - to develop an online game?

Javascript’s main use for us is to create an interface for the game. Through Javascript we can capture user input - i.e. mouse clicks and text. We can also alter the game’s output - changing the HTML of the page, adding images, and moving things around. Javascript will provide a lot of the front end work.

PHP, on the other hand, is a more robust language for dealing with the logic of the game. PHP could be useful for developing an AI and evaluating winning conditions. It also offers a great way to store information for later use through flat files or database integration.

If we’re going to use both Javascript and PHP, we’ll need to use AJAX. This is the glue that holds the whole thing together. It will send input from the main Javascript to the PHP processing scripts. The PHP script will then send info back to the Javascript and it will alter the page’s layout accordingly.

Read the full first tutorial here.

The second tutorial goes into more detail as to how you would start creating a simple game. By the end of the tutorial you have a complete simple tic-tac-toe game (the game needs more logic added to be finished, but the concept is there).

Below is an excerpt from the second tutorial.

Creating Graphics

Tic Tac Toe Circle ImageI created two images in Inkscape to use for this board. Each is a 50×50 png file - with a gray background and a black border. The actual token (the circle or cross) is red and laid on top of the gray background.

Tic Tac Toe Cross ImageI’m no great artist, but these should work for a functional demo. We can worry about making nice circle and cross tokens later. I think getting the game working is more important than making it pretty from the get-go.

Turning Our .js Script into a Class

In making this a functional board, I also converted the script into an actual class. The script may end up being somewhat large and unwieldy - and an object oriented approach may help us keep it tidy and clean. Or it may add a lot of overhead… but I like objects.

In our HTML file, we’ll create the object like this.

<script type="text/javascript">
var tictactoe = new game();
</script>

In the attached .js file, we actually define the game object. Here’s part of the object definition.

function game() {
  //  Array to hold the bgImgs
  this.bgImgs = new Array();
    this.bgImgs[0] = 'tttcircle.png';
    this.bgImgs[1] = 'tttcross.png';

 
  //  Player information			
  this.currentPlayer = 0;
  this.players = new Array();
    this.players[0] = "Player One";
    this.players[1] = "Player Two";

 
   return true;

This class constructor does some of the initialization for us.

First, it creates an array with background images. At the moment we’re only using two images. However, this technique would be useful if you had a more complex map - with 10-15 images you could lay over a div.

Second, we create some player information variables. The ‘currentPlayer’ property is going to track whether the ‘cross’ or ‘circle’ player is currently taking a turn. The ‘players’ array will just hold the names of those players for now.

The Background Image Changing Function

One of the major methods of this class will be changeBackground(). Just like in the previous example, this method will change the background style of a given div tag. This way we can change it from an open square to a circle or cross token.

  this.changeBackground = function (boxId) {
    var box = document.getElementById('box-' + boxId);
    box.style.background = 'transparent url(' + 
      this.bgImgs[this.currentPlayer] + ') top left no-repeat';

 
    box.removeAttribute('onClick');
 
    this.changePlayer();
  }

This should be pretty straightforward. We’re storing the ‘div’ element in the ‘box’ variable. We’re then setting the ‘background’ style as we would in a css style. Remember that we stored the background images in an array (this.bgImgs) and this.currentPlayer corresponds to a key in the this.bgImgs array (either 0 or 1).

The ‘box.removeAttribute()’ method is removing ‘onClick’ from that div. We can’t use a square a second time, so we might as well eliminate the onClick handler altogether.

Finally, this.changePlayer() is calling a new method. This is going to help us switch from Player One’s turn to Player Two’s turn.

One Turn to Another - this.changePlayer()

The last method we need to declare for this class at the moment is changePlayer.

This will toggle the active player - which in turn affects whether a circle or cross is placed on the board. For some added effect, we’ll also create a new html element to display a message that says who’s turn it is.

  this.changePlayer = function () {
    //  Switch the active player

    if (this.currentPlayer == 0) {
      this.currentPlayer = 1;
    } else {

      this.currentPlayer = 0;
    }
 
    //  Get a reference to our 'message' element and create the message
    var box = document.getElementById('message');
    var msg = "It is " + this.players[this.currentPlayer] + "'s turn.";
    var txt = document.createTextNode(msg);

 
    //  Erase any existing text
    while (box.hasChildNodes()) {
      box.removeChild(box.lastChild);
    }

 
    //  Add the text node (our message) to our element
    box.appendChild(txt);
  }

Again, this is pretty straightforward. The DOM functions are amazingly simple - once you see how they work.

‘box’ is a reference to our element (id = message). The msg variable is a temp variable I created to hold the string. The ‘createTextNode’ method creates a new block of text (with our msg) that we can then insert into an HTML element.

The while() loop is simply there to erase any old text. As long as our ‘box’ element has any child nodes inside of it (text or other HTML tags), the loop will execute and delete one of those child nodes each time. This way we have a clean slate on which to write down who’s turn it is.

Read the full second tutorial here.

Although the tutorial only shows a simplistic tic-tac-toe game you could use the concepts as a starting point to build your own web games. As always if you have made any cool games we would love to hear about them either in the comments or you can write a blog post about it using your free Ajaxonomy account.

[...] Creating a Simple JavaScript Game Board By David Hurth Over at the Web Cash blog they have posted two tutorials on Game development in JavaScript, Ajax and PHP. The first tutorial explains the concept and describes how to make a game board. Below is an excerpt from the first tutorial. … Ajaxonomy - The Study of Ajax… - http://www.ajaxonomy.com [...]

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