Cannonball and Slingshot - The Essential Guide to HTML5 - page 121

Code Explanation

function Ball(sx,sy,rad,stylestring) {

this.sx = sx;

this.sy = sy;

this.rad = rad;

this.draw = drawball;

this.moveit = moveball;

this.fillstyle = stylestring;

}

function drawball() {

ctx.fillStyle=this.fillstyle;

ctx.beginPath();

//ctx.fillStyle= rgb(0,0,0);

ctx.arc(this.sx,this.sy,this.rad,
0,Math.PI*2,true);

ctx.fill();

}

function moveball(dx,dy) {

this.sx +=dx;

this.sy +=dy;

}

var cball = new Ball(cannonx+cannonlength,
cannony+cannonht*.5,ballrad,"rgb(250,0,0)");