Having the knowledge of moving images using JavaScript, we’ll be creating a small Ping Pong game as an example for this post.
Today we’ll learn to do a few new things in JavaScript:
1. Executing some code every specified time interval (for game loop).
2. Tracking and using mouse movements.
3. Attaching code (function) to events.
Game Theory
As you should be knowing, in this game there is one ball and two paddles at two ends. One is user-controlled the other, for this example, is CPU controlled. User has to move the paddle in order not to let the ball pass through, CPU also has to do the same thing. Whoever’s side the ball passes through looses the game.
There are a few objects which can interact with each other, these are ball, paddles, walls. Let’s see the various interactions that can take place between these:
Ball Hitting Upper/Lower Wall – Ball will bounce off.
Ball Passing Through Either Side – Player or CPU, depending on whose side ball passed through, will loose the game.
Ball Hitting Paddle – It’ll bounce off
We’ll need to take care of these events:
Page Load – Game objects will be initialized
Game Start – Mouse click on the player paddle will start the game.
Mouse Movements – Inside the game area (a div), the paddle will follow the y-position of the mouse. Paddle however should not get past the boundaries.
CPU Paddle – The paddle will follow the ball by moving up/down depending the relative position of the ball. We’ve added a little intelligence by only moving the paddle while ball is coming towards it. This will make the movement as well as the game look more real.
Code
NOTE: Put two files ball_small.png, paddle.png (Right-Click "Save As") in the same directory the script is in.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Pong Game In JavaScript</title>
<style>
#box
{
width: 500px;
height: 300px;
margin: auto;
border: 5px solid #ccc;
position: relative;
overflow: hidden;
}
.ob
{
position: absolute;
border: 0px;
}
</style>
<script type="application/javascript">
// CHANGE THESE, IF REQUIRED
var Speed = 5; // Speed of ball (pixels/step)
var CPUSpeed = 5; // Speed of CPU Paddle (pixels/step)
// Short references to objects
var paddle1;
var paddle2;
var ball;
var box;
var msg;
// For internal use
var dx, dy; // Speed in x and y directions
var ballX, ballY; // x and y positions of ball
var playerY; // y position of player paddle (x fixed)
var cpuY; // y position of CPU paddle (x fixed)
var iID; // To store ID of set interval used to clear it when required
// Attach a function to onLoad event
window.onload = Init;
// INITIALIZE GAME OBJECTS
function Init()
{
// Make short refrences to objects
paddle1 = document.getElementById(paddle1);
paddle2 = document.getElementById(paddle2);
ball = document.getElementById(ball);
box = document.getElementById(box);
msg = document.getElementById(msg);
// Initial values
ballX = (box.offsetWidth / 2) - (ball.offsetWidth / 2);
ballY = (box.offsetHeight / 2) - (ball.offsetHeight / 2);
cpuY = (box.offsetHeight / 2) - (paddle2.offsetHeight / 2);
playerY = (box.offsetHeight / 2) - (paddle1.offsetHeight / 2);
dx = dy = Speed;
paddle1.style.left = 20 + px;
paddle1.style.top = playerY + px;
paddle2.style.left = box.offsetWidth - (20 + paddle2.offsetWidth) + px;
paddle2.style.top = cpuY + px;
ball.style.left = ballX + px;
ball.style.top = ballY + px;
// Show message
msg.innerHTML = <h2>Click on Paddle to Start Game.</h2>;
}
// START GAME
function Start()
{
// Attach a function to onmousemove event of the box
box.onmousemove = MovePaddle;
// Call GameLoop() function every 10 milliseconds
iID = setInterval(GameLoop(), 10);
msg.innerHTML = ;
}
// MAIN GAME LOOP, CALLED REPEATEDLY
function GameLoop()
{
// MOVE BALL
ballX += dx;
ballY += dy;
// See if ball is past player paddle
if(ballX < 0)
{
clearInterval(iID);
Init();
box.onmousemove = ;
msg.innerHTML = <h2>You Loose!<br/>Click on Paddle to Re-Start Game.</h2>;
}
// See if ball is past CPU paddle
if((ballX + ball.offsetWidth) > box.offsetWidth)
{
clearInterval(iID);
Init();
box.onmousemove = ;
msg.innerHTML = <h2>You Win!<br/>Click on Paddle to Re-Start Game.</h2>;
}
// COLLISION DETECTION
// If ball hits upper or lower wall
if(ballY < 0 || ((ballY + ball.offsetHeight) > box.offsetHeight))
dy = -dy; // Make x direction opposite
// If ball hits player paddle
if(ballX < (paddle1.offsetLeft + paddle1.offsetWidth))
if(((ballY + ball.offsetHeight) > playerY) && ballY < (playerY + paddle1.offsetHeight))
dx = -dx;
// If ball hits CPU paddle
if((ballX + ball.offsetWidth) > paddle2.offsetLeft)
if(((ballY + ball.offsetHeight) > cpuY) && ballY < (cpuY + paddle2.offsetHeight))
dx = -dx;
// Place ball at calculated positions
ball.style.left = ballX + px;
ball.style.top = ballY + px;
// MOVE CPU PADDLE
// Move paddle only if ball is coming towards the CPU paddle
if(dx > 0)
{
if((cpuY + (paddle2.offsetHeight / 2)) > (ballY + ball.offsetHeight)) cpuY -= CPUSpeed;
else cpuY += CPUSpeed;
paddle2.style.top = cpuY + px;
}
}
// TO MOVE PLAYER PADDLE ON MOUSE MOVE EVENT
function MovePaddle(e)
{
// Fetch y coordinate of mouse
var y = (e.clientY - (box.offsetTop - document.documentElement.scrollTop));
// Here, (box.offsetTop - document.documentElement.scrollTop) will get the relative
// position of "box" w.r.t to current scroll postion
// If y below lower boundary (cannot go above upper boundary -
// mousemove event only generated when mouse is inside box
if(y > (box.offsetHeight - paddle1.offsetHeight))
y = (box.offsetHeight - paddle1.offsetHeight);
// Copy position
playerY = y;
// Set position
paddle1.style.top = y + px;
}
</script>
</head>
<body bgcolor="#fff">
<h1 align="center">Pong Game Example in JavaScript</h1>
<div id="box">
<img class="ob" id="paddle1" src="paddle.PNG" onclick="javascript: Start()"/>
<img class="ob" id="paddle2" src="paddle.PNG" />
<img class="ob" id="ball" src="ball_small.PNG" />
</div>
<div id="msg" align="center"></div>
</body>
</html>
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