Membuat Popup HTML dengan Javascript

Saya akan berikan fungsi javascript untuk popup di HTML

 function pop_up() //fungsi popup   
 {  
   var width = 800;  
   var height = 400;  
   var left  = (screen.width - width)/2;  
   var top  = (screen.height - height)/2;  
   var params = 'width='+width+', height='+height+',scrollbars=yes';  
   params += ', top='+top+', left='+left;  
   window.open('form_popup.php','Form Popup',params);  
 };