Module not found in angularjs

I am new to javascript and have spent a couple of hours to discover my issue. The module initialization function can be ignored. To avoid this, do not forget to add empty parenthesis to the end of a function:

(function() {
    "use strict";
    var app = angular.module("app", []);
    })(); //<<---Here

Many has fallen into the same trap. Me included.

The following does not define a new module. It will try to retrieve a module named UserHandler which isn't defined yet.

angular.module('UserHandler')

Providing a (empty) array of dependencies as the second argument will define your module.

angular.module('UserHandler', [])