March 16, 2010

Diving into JavaScript with Tetris

To prepare for the Devnology meeting next month I have to write a version of Tetris in a language of my choosing. I had already fired up Visual Studio to build a kick-ass C# XNA version with explosions and 3D visuals when I realized it would be much more fun and educational to pick a language I’m less familiar with. I’m pretty sure there will be Ruby, Python, Java and Haskell implementations by some people who are pretty good at programming in those languages so I decided not to pick any of those obvious languages. But I’d still like to invest my time learning something I’m actually going to use, so Cobol wasn't an option.

I picked JavaScript.

The next couple of posts I’m going to be building Tetris using JavaScript and jQuery. I’ll pick a couple of stories every few days to implement and if I’m lucky I’ll have something resembling Tetris in a couple of weeks to show off. I actually hope I’ll get some feedback when I do things wrong. Most of my JavaScript experience has been adding simple onclick handlers to html files and pulling my hair out because of DOM incompatibilities. The new JavaScript frameworks hide all the DOM ugliness so it’s a good time to dive into JavaScript again.

In this first post we’ll get started implementing a couple of easy stories. I chose the following three.

  • As a user I can see a playing field that is 10 units wide and 18 units high.
  • Blocks fall one unit every second.
  • As a user I can steer blocks left and right with the arrows on the numeric keypad.

Lets get started.

As a user I can see a playing field that is 10 units wide and 18 units high

The first story is too easy. The display is just going to be a div. So we create a tetris.html file like this:

   2: <html>
   3:     <head>
   4:         <link rel="stylesheet" href="tetris.css" />
   5:     </head>
   6:     <body>
   7:         <div id="field"></div>
   8:     </body>
   9: </html>

and the stylesheet:

   1: #field
   2: {
   3:     position: relative;
   4:     top: 40px;
   5:     left: 50%;
   6:     margin-left: -100px;
   8:     width: 200px;
   9:     height: 360px;
  11:     border: solid grey 2px;
  12: }

I decided on a block size of 20 pixels so the field is 200 by 360 pixels.

Blocks fall one unit every second

For this story to work we first need to be able to draw blocks. jQuery makes this pretty easy. Especially since blocks are just going to be small div’s with a big border in this first version, so our first pieces of JavaScript look like this:

   1: function drawBlock(x, y) {
   2:     var block = $("<div class='block'></div>").css('top', (17 - x) * 20).css('left', y * 20);
   3:     $('#field').append(block);
   4: }


   1: function clearBlocks() {
   2:     $('.block').remove();
   3: }

In order to draw a block every second we need some sort of gameloop that is called every second. So I put something like this in the jQuery document ready handler :

   1: $(document).ready(function() {
   2:     setInterval( "gameLoop()", 1000);
   3: });

and the gameloop:

   1: var verticalPosition = 18;
   2: var horizontalPosition = 5;
   4: function gameLoop() {
   5:     verticalPosition--;
   6:     clearBlocks();
   7:     drawBlock(verticalPosition, horizontalPosition);
   8: }


some css for the blocks:


   1: div.block
   2: {
   3:     position: absolute;
   5:     width: 12px;
   6:     height: 12px;
   7:     border: solid black 4px;
   8: }


And voila, we’ve got movement on our screen. Pretty easy actually.

As a user I can steer blocks left and right with the arrows on the numeric keypad.

We still can’t call this a game though. Movement is nice but without interactivity we might as well be looking at dancing hamsters. We want to be able to steer our block around.

jQuery makes registering a keyboard handler pretty easy. We can just add:

   1: $(document).keydown(keyHandler);

and the handler:


   1: const keyCodeMoveLeft = 100;
   2: const keyCodeMoveRight = 102;
   3: const keyCodeMoveDown = 98;
   5: function keyHandler(keyEvent) {
   7:     if(keyEvent.keyCode === keyCodeMoveLeft) {
   8:         horizontalPosition--;
   9:     }
  11:     if(keyEvent.keyCode === keyCodeMoveRight) {
  12:         horizontalPosition++;
  13:     }
  15:     if(keyEvent.keyCode === keyCodeMoveDown) {
  16:         verticalPosition--;
  17:     }
  19:     clearBlocks();
  20:     drawBlock(verticalPosition, horizontalPosition);
  21: }

Next time we’re going to look at tooling to make testing and debugging all this a bit easier and we’re going to get a bit more OO.

You can download the code in this post here:

tetris0.01.rar (1.02 kb)

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