Document The current browser does not support Canvas, can replace the browser a try! { x : 400, y : 400, radius : 150, vx : Math.random() , vy : Math.random() } window.onload = function(){ ); canvas.width = 1024; canvas.height = 768; )){ ); setInterval(function(){ draw(context); update(canvas.width,canvas.height); },40); }else{ alert(); } } function draw(cxt){ cxt.canvas; cxt.clearRect(,canvas.width,canvas.height); cxt.save(); cxt.beginPath(); cxt.fillStyle ; cxt.fillRect(,canvas.width,canvas.height) cxt.closePath(); cxt.beginPath(); cxt.arc(searchLight.x,searchLight.y,searchLight.radius,); cxt.fillStyle ; cxt.fill(); cxt.clip(); cxt.closePath(); cxt.font ; cxt.textAlign ; cxt.textBaseline ; cxt.fillStyle ; cxt.fillText(); cxt.fillText(); cxt.fillText(); cxt.restore(); } function update(canvasWidth,canvasHeight){ searchLight.x += searchLight.vx; searchLight.y += searchLight.vy; ){ searchLight.vx = -searchLight.vx; searchLight.x = searchLight.radius; } canvasWidth){ searchLight.vx = -searchLight.vx; searchLight.x searchLight.radius; } ){ searchLight.vy = -searchLight.vy; searchLight.y = searchLight.radius; } canvasHeight){ searchLight.vy = -searchLight.vy; searchLight.y searchLight.radius; } }