var pop //initialize as global variable so that it will only have a null value once

function popUp(URL,width,height,location,status,top,left)
{
/* 
if we test closed property when window has not been opened (null), the script will die here. So before testing closed property, make sure window has been opened(does not have a null value) 
*/ 
if(pop!=null && !pop.closed)
{
pop.close()//if pop is opened, close it before running open method in order to re-assign properties
}
pop = window.open(URL,"win1","width="+width+",height="+height+",location="+location+",status="+status+",top=" +top+",left="+left+",resizable=1")
pop.focus()  // forces the popup to come to the front if it has already been opened, but not closed
}