Chrome extension, javascript: Tab giving me an error -

this simple function:

function do_the_redirect(where_to) {                  up2 = new object();                 up2.url =where_to;                 //alert("here");                 chrome.tabs.update(tab.id, up2);  } 

but gives me error:

uncaught referenceerror: tab not defined 

// in background.html

function onrequest(request, sender, sendresponse) {     function do_the_redirect(where_to) {         var up2 = new object();         up2.url = where_to;         chrome.tabs.update(sender.tab.id, up2);    }; }; chrome.extension.onrequest.addlistener(onrequest);