运动的小球

  1 <!DOCTYPE html>
  2 <html>
  3 <head>
  4 <meta charset="utf-8">
  5 <title></title>
  6 <style type="text/css">
  7 #box{width: 500px;height: 500px;border: 1px solid black;position: relative;}
  8 #ball{width: 20px;height: 20px;background-color: red;border-radius: 50%;position: absolute;left: 0;top: 0;}
  9 </style>
 10 </head>
 11 <body>
 12 <div id="box">
 13 <div id="ball">
 14 
 15 </div>
 16 </div>
 17 <input type="button" name="btn" id="btn" value="启动" />
 18 </body>
 19 <script type="text/javascript">
 20 //获取节点
 21 var obox = document.querySelector("#box");
 22 var oball = document.querySelector("#ball");
 23 var obtn = document.querySelector("#btn");
 24 var speed = 10 ;
 25 var topspeed = 0 ;
 26 var t ;
 27 obtn.onclick = function(){
 28 clearInterval(t);
 29 t = setInterval(function(){
 30 
 31 if(oball.offsetLeft>480 ){
 32 speed=-speed;
 33 oball.style.left = oball.offsetLeft + speed + "px";
 34 }
 35 else if (oball.offsetLeft<0 ){
 36 speed=-speed;
 37 oball.style.left = oball.offsetLeft + speed + "px";
 38 }else{
 39 oball.style.left = oball.offsetLeft + speed + "px";
 40 }
 41 
 42 
 43 if(oball.offsetTop>480 ){
 44 topspeed=-topspeed;
 45 oball.style.top = oball.offsetTop + topspeed + "px";
 46 }
 47 else if (oball.offsetTop<0 ){
 48 topspeed=-topspeed;
 49 oball.style.top = oball.offsetTop + topspeed + "px";
 50 }else{
 51 oball.style.top = oball.offsetTop + topspeed + "px";
 52 }
 53 
 54 
 55 document.onkeydown = function(eve){
 56 e=eve||window.event;
 57 //左走
 58 if(e.keyCode==37){
 59 if(speed>0){
 60 
 61 speed=-10;
 62
 63 }else if(speed==0){
 64 speed=-10;
 65 topspeed=0;
 66 }
 67 }
 68 //右走
 69 if(e.keyCode==39){
 70 if(speed<0){
 71 speed=-speed;
 72 
 73 }else if(speed==0){
 74 speed=10;
 75 topspeed=0;
 76 }
 77 }
 78 
 79 //    上走
 80 if(e.keyCode==38){
 81 if(topspeed>0){
 82 topspeed=-10;
 83 
 84 }else if(topspeed==0){
 85 topspeed=-10;
 86 speed=0;
 87 
 88 }
 89 }
 90 
 91 //    下走
 92 if(e.keyCode==40){
 93 if(topspeed<0){
 94 topspeed=10;
 95 
 96 }else if(topspeed==0){
 97 topspeed=10;
 98 speed=0;
 99 
100 }
101 }
102 
103 }
104 
105 },30)
106 
107 }
108 </script>
109 </html>
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