window.open窗口显示在屏幕中央

function showNew()

{

var width=200;

    var height=200;

var y=(window.screen.availHeight-200);

var x=(window.screen.availWidth-200);

var mywindow=window.open("","_blank","height="+height+",width="+width);

mywindow.moveTo(x/2,y/2);

}

</script>

<input type="button" value="点击" onclick="showNew()">