<div id=div1 style="display:block;width=200;height=200;background:#222222;"></div>
<input type=button onclick=show()>
<script>
var showmode=true;
function show(){
  showmode=(showmode)?false:true;
  if(showmode){
    div1.style.display='block';
 }
  else{
    div1.style.display='none';
 }
 }
</script>