You have to select "ProfStef next." text on the page, in a manner browser's document.getSelection would find those (not sure whenever this is possible on iPhone

You have to select "ProfStef next." text on the page, in a manner browser's document.getSelection would find those (not sure whenever this is possible on iPhone). I think, given this is a newbie-oriented tutorial, the alert should be more helpful, like "empty program, select some text first!" instead of unexpected null character.