js同时按下两个方向键

<SCRIPT LANGUAGE="JavaScript"> <!-- var dn=0; //1 up, 2 left, 3 both document.onkeyup = function() {   if(event.keyCode== 37)   {     if(dn==2)     {       dn=0     }   else if(dn==3){dn=1}   }   if(event.keyCode== 38)   {     if(dn==1){dn=0}     else if(dn==3){dn=2}   } } document.onkeydown = function() {   if(event.keyCode== 37)   {     ren.innerText="aa"//向左     if(dn==0){dn=2}     else if(dn==1){dn=3}   }   else if(event.keyCode==38)   {     ren.innerText="bb"//向上   if(dn==0){dn=1}   else if(dn==2){dn=3}   }   if(dn==3)   {     dn=0     alert("cc")   } } //--> </SCRIPT> <div id=ren></div>