var pop //create global variable without assigning a value so that its value is null
function popUp(URL,w,h,y,x){

if(pop!=null && !pop.closed){ // if popUp has been opened, and popUp hasn't already been closed
pop.close() //close it in order to re-assign properties
}
pop = window.open(URL,"win1","width="+w+",height="+h+",resizable="+y+",scrollbars="+x+",location=0")
pop.focus() 
}