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>