I get an error saying that "mainCtrl" is not a function and thus is undefined.

If you post your code, or make a snapshot of your workspace and post the link, someone could look and perhaps identify and explain the issue.

Did you find an answer to your problem, Nina? Let us know, and be sure to include your code, as Steven mentioned, if you need more help. Thanks!

Change .controller to .controller('mainCtrl',['$scope',function($scope) { $scope.helloWorld = function() { console.log('This is the helloWorld controller function'); }; }]);

I had the same error: angular.js:12416 Error: [ng:areq] Argument 'mainCtrl' is not a function, got undefined

Here is the code:

angular.module("todoListApp", [])
.controller('mainCtrl', function($scope){
  $scope.helloWorld = function() {
    console.log("Hello there! this is the controller function.");
  };
});

That code works for me, stevenorton. Perhaps if you share the contents of your HTML file, I could troubleshoot further. Also, are you in workspaces? Or are you working on your local machine?