How can I include address bar variable in this javascript line?

How can I include address bar variable in this javascript line?

Can someone please tell me how I can add in the address bar variable to my
javascript?
The website address reads: www.example.com/index.php?user=74
and the javascript that needs to include 'user = 74' is:
xmlhttp.open("GET","../profile/" + pageName + ".php",true);