gistfile1.js

var updatePeriod = 5 * 60 * 1000; //mins * seconds * 1000
var locationUpdateEnabled = false;

// This gets fired when the webView has loaded.
// Rather than using the generic event of the webview 
// - it's more reliable to create your own and have the page itself trigger when it's ready
// - This allows your code to initialise things properly before asking for update
Ti.App..addEventListener('pageReadyForUpdates', function(e) {
  // This will only get fired once and when the page itself is ready to start receiving update
  locationUpdateEnabled = true;
  updateLocation();
});

function updateLocation() {
    // Store the value in a placeholder - so we don't lose any prev data
    var cityFound = getCity();
    if (cityFound.error) {
      // Deal with the error
      // Here we can stop future checks for updates
      locationUpdateEnabled = false;
      // Note the interval timer is still running - if you want to stop this also then
      // clearInterval(triggerUpdate);
    } else {
      // We have valid data
      city = cityFound;
      data = new Date();
      // fire the event into the webview - 
      // but let's send it to an event listener that's specific to receiving updates to the location
      // You'll need an eventListener in the page to pick this up
      Ti.App.fireEvent('pageUpdateLocation', {data:data, city:city});
    }
}

var triggerUpdate = setInterval(function(e){
  if (locationUpdateEnabled) {
    updateLocation();
  }
}, updatePeriod);