js计时器clearInterval的清除

document.onkeydown = function(e){
 var timer1,timer2,timer3,timer4;
改成

// 不要每次执行时都要初始化计数器
var timer1,timer2,timer3,timer4;
document.onkeydown = function(e){
定义一个全局方向标志，例如 var direction = null;

function xia(){
 if(direction == 'xia') return false;
 direction = 'xia';
 document.getElementById("object").style.top = (parseInt(document.getElementById("object").style.top)+ 10) + "px";
  if(parseInt(document.getElementById("object").style.top) > 490){
   document.getElementById("object").style.top = 490 + "px";
  }
}