Hi, i did the step by step exercises of the chapter. And as suggested by the exercise (question no 11. and 12.), i try to step through the init function, there is an error message "Uncaught ReferneceError: contactScreen is not defined", may i know which part actually when wrong? or is there anything went wrong? i just can not remember where contactScreen is defined too?

* i copied the source file from the website for chapter 14, it is the same.

for easy reference:
<script>
var mainElement = document.getElementById('contactScreen');
var appScreen = contactsScreen(mainElement);
appScreen.init();
</script>


Thanks in advance