CODE EXAMPLE FOR JAVASCRIPT

chrome.tab.onupdated

chrome.tabs.onUpdated.addListener(function(tabId, changeInfo, tab) {
   alert(changeInfo.url);
}); 

chrome.tabs.onActivated.addListener(function(activeInfo) {
  // how to fetch tab url using activeInfo.tabid
  chrome.tabs.get(activeInfo.tabId, function(tab){
     console.log(tab.url);
  });
});