I’m writing a little object oriented style javasscript demo — just to draw a

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Asked: May 29, 20262026-05-29T11:33:32+00:00 2026-05-29T11:33:32+00:00

I’m writing a little object oriented style javasscript demo — just to draw a

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I’m writing a little object oriented style javasscript demo — just to draw a bunch of balls moving around the screen. nothing fancy, no collision detection or anything at this point. Consider it safe to assume my Ball.js class is good.

My question amounts to this: Where should I call ball.draw(context) ? The only way to get balls drawn to the screen the way I set it up seems to be by placing the call in generateBalls(). But that means each ball is just drawn once.

So I’d really appreaciate it if someone could point out the error of my ways here. This isn’t homework – just trying to get a better handle on javascript and canvas.

<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>Untitled Document</title>
<script src="ball.js"></script>
<script src="utils.js"></script>
...
    <canvas id="canvas" width="600" height="480"></canvas>
    <script type="text/javascript">
        window.addEventListener('load', eventWindowLoaded, false);

        function eventWindowLoaded() {
            canvasApp();    
        }

        function canvasSupport() {
            return true;    
        }

        function canvasApp() {
            if(!canvasSupport()) {
                return; 
            }
        }
        console.log("app entered");
        var numBalls = 45;
        //var numBalls = demo.numberofballs.value;
        var maxSize = 8;
        var minSize = 5; 
        var maxSpeed = maxSize + 5;
        var balls = new Array();
        var tempBall;
        var tempX;
        var tempY;
        var tempSpeed;
        var tempAngle;
        var tempRadius;
        var tempRadians;
        var tempXunits;
        var tempYunits;

        canvas = document.getElementById("canvas");
        context = canvas.getContext("2d");

        generateBalls();

        setInterval(drawScreen, 33);

        function generateBalls() {
            console.log("Make some balls");
            for(var index = 0; index < numBalls; index++) {
                var tempRadius = Math.floor(Math.random()*maxSize)+minSize;
                var ball = new Ball(tempRadius, "#000000"); 
                ball.x = tempRadius * 2 + (Math.floor(Math.random()*canvas.width) -  tempRadius * 2);
                ball.y = tempRadius * 2 + (Math.floor(Math.random()*canvas.height) - tempRadius * 2);
                ball.speed = maxSpeed - tempRadius;
                ball.angle = Math.floor(Math.random()*360);
                ball.dx = Math.cos(tempRadians) * tempSpeed;
                ball.dy = Math.sin(tempRadians) * tempSpeed;
                // here outputted balls but a stupid place to put it LOL
                balls.push(ball);



            }

        }

        function drawScreen() {
            console.log("draw screen");



            // loop through all balls and adjust their position
            // a BallManager could do this more cleanly
            for(var index = 0; index < balls.length; index++) {

                context.fillStyle="#EE00EE";
                context.fillRect(0,0,canvas.width, canvas.height);

                // Box
                context.strokeStyle = "#ff0043";
                context.strokeRect(1,1,canvas.width-2, canvas.height-2);

                // place balls
                context.fillStyle = "#ff8783";
                console.log("ball mover loop in drawscreen");
                // no var ball now

                ball = balls[index];
                ball.x += ball.dx;
                ball.y += ball.dy;
                ball.draw(context);
                //checkBoundaries(balls[index]);
                if(ball.x > canvas.width || ball.x < 0) {
                ball.angle = 180 - ball.angle;
                updateBall(ball);   
                    } else if(ball.y > canvas.height || ball.y < 0) {
                        ball.angle = 360 - ball.angle;
                        updateBall(ball);   
                        //ball.draw(context);
                }

            }

        }   

        //function checkBoundaries(ball) {
            //console.log("Check Bounds: " + " " + "ball.x: " + ball.x + " " + //"ball.y: " + ball.y);

        //}

        function updateBall(ball) {
            ball.radians = ball.angle * Math.PI / 180;
            ball.dx = Math.cos(ball.radians) * ball.speed;
            ball.dy = Math.sin(ball.radians) * ball.speed;
            //ball.draw(context);
        }

    </script>
</body>
</html>

Thank you for your advice,
Marc

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1 Answer

Editorial Team · Answer 1 · 2026-05-29T11:33:34+00:00

Your example contains more than one error, please check your modified code. It works, but you must extend and correct it.

<!DOCTYPE HTML>
<html>
  <head>
  <meta charset="utf-8">
  <title>Untitled Document</title>
  <script type="text/javascript">
    // next lines is a Ball() implementation code
    Ball = function(radius,color) {
        this.radius=radius;
        this.color=color;
    };
    Ball.prototype.x=0;
    Ball.prototype.y=0;
    Ball.prototype.speed=0;
    Ball.prototype.angle=0;
    Ball.prototype.dx=0;
    Ball.prototype.dy=0;
    Ball.prototype.radius=10;
    Ball.prototype.color="#000";
    Ball.prototype.draw=function() {

        context.beginPath();
        context.arc(this.x,this.y,this.radius,0,Math.PI*2,true);
        context.lineWidth = 5;
        context.strokeStyle = this.color; // line color
        context.stroke();
        context.closePath();
    };

    window.addEventListener('load', eventWindowLoaded, false);

    function eventWindowLoaded() {
        canvasApp();    

        //console.log("app entered");
        window.canvas = document.getElementById("canvas");
        window.context = canvas.getContext("2d");

        generateBalls();
        // if you want to use setInterval() instead replace next line
        setTimeout(drawScreen, 33);
    }

    function canvasSupport() {
        return true;    
    }

    function canvasApp() {
        if(!canvasSupport()) {
            return;
        }
    }

    var numBalls = 45;
    //var numBalls = demo.numberofballs.value;
    var maxSize = 8;
    var minSize = 5;
    var maxSpeed = maxSize + 5;
    var balls = new Array();
    var tempBall;
    var tempX;
    var tempY;
    var tempSpeed;
    var tempAngle;
    var tempRadius;
    var tempRadians;
    var tempXunits;
    var tempYunits;

    function generateBalls() {
        //console.log("Make some balls");
        for(var index = 0; index < numBalls; index++) {
            var tempRadius = Math.floor(Math.random()*maxSize)+minSize;
            var tempRadians = Math.random()*Math.PI;
            var tempSpeed = 10;
            var ball = new Ball(tempRadius, "#000000");
            ball.x = tempRadius * 2 + (Math.floor(Math.random()*canvas.width) -  tempRadius * 2);
            ball.y = tempRadius * 2 + (Math.floor(Math.random()*canvas.height) - tempRadius * 2);
            ball.speed = maxSpeed - tempRadius;
            ball.angle = Math.floor(Math.random()*360);
            ball.dx = Math.cos(tempRadians) * tempSpeed;
            ball.dy = Math.sin(tempRadians) * tempSpeed;
            // here outputted balls but a stupid place to put it LOL
            balls.push(ball);
        }
    }

    function drawScreen() {
        console.log("draw screen");


        context.fillStyle="#EE00EE";
        context.fillRect(0,0,canvas.width, canvas.height);

        // Box
        context.strokeStyle = "#ff0043";
        context.strokeRect(1,1,canvas.width-2, canvas.height-2);

        // loop through all balls and adjust their position
        // a BallManager could do this more cleanly
        for(var index = 0; index < balls.length; index++) {
            // place balls
            context.fillStyle = "#008700";
            //console.log("ball mover loop in drawscreen");
            // no var ball now

            ball = balls[index];
            ball.x += ball.dx;
            ball.y += ball.dy;
            ball.draw(context);
            //checkBoundaries(balls[index]);
            if(ball.x > canvas.width || ball.x < 0) {
                ball.angle = 180 - ball.angle;
                updateBall(ball);   
            } else if(ball.y > canvas.height || ball.y < 0) {
                ball.angle = 360 - ball.angle;
                 updateBall(ball);   
                //ball.draw(context);
            }

        }
        // if you want to use setInterval() instead remove next line
        setTimeout(drawScreen, 33);
    }   

    //function checkBoundaries(ball) {
        //console.log("Check Bounds: " + " " + "ball.x: " + ball.x + " " + //"ball.y: " + ball.y);

    //}

    function updateBall(ball) {
        ball.radians = ball.angle * Math.PI / 180;
        ball.dx = Math.cos(ball.radians) * ball.speed;
        ball.dy = Math.sin(ball.radians) * ball.speed;
        //ball.draw(context);
    }

  </script>
  </head>
  <body>
    <canvas id="canvas" width="600" height="480" style="background:red;"></canvas>
  </body>
</html>

http://jsfiddle.net/QVgZx/2/

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