How about keep it simple and just show a loading screen without progress indication that disappears when everything has loaded? Check out the following topic for more information and a code example: Basic Loading Screen
I personally doubt the value of a progress bar. If you still want one, you can compute the current progress in percent ([0,1]) via itemsLoaded / itemsTotal. This value can be used to modulate a progress bar implemented via CSS. How this value is applied depends on the markup/styles.