js跑步算法

 <!DOCTYPE html>
 <html>
 <head>
 <title>JavaScript</title>
 <style>
 html
 {
 background-color:silver;
 }
 .point1
 {
 position:absolute;
 left:10px;
 top:40px;

 }
 .point2
 {
 position:absolute;
 left:100px;
 top:40px;
 }
 .hr1
 {
 position:absolute;
 top:60px;
 }
 </style>
 <script type="text/JavaScript">
 document.onmousedown = mousedown;
 document.onmouseup = mouseup;
 var intervalProcess;
 var direct = true;
 function mousedown(){
 intervalProcess = setInterval("MovePoint()", 1);
 }
 function mouseup(){
 clearInterval(intervalProcess);
 }
 function MovePoint(){
 with (document.getElementById("point1").style){
 if (isNaN(parseInt(left)))
 left = "10px";
 else {
 document.getElementById("point2").style.left = "200px";
 if (parseInt(left) < 0)
 direct = true;
 if (parseInt(left) > parseInt(document.getElementById("point2").style.left))
 direct = false;
 if (direct)
 left = parseInt(left) + 1 + "px";
 else
 left = parseInt(left) - 1 + "px";
 }
 }
 }
 </script>
 </head>
 <body>
 <div class="point1" id="point1"><font color=blue>a</font></div>
 <div class="point2" id="point2"><font color=red>b</font></div>
 <hr class="hr1" />
 </body>
 </html