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JavaScript AngularJS Basics (1.x) Services in Angular Using Services To Get Data

*** SOLVED*** ReferenceError: callback is not defined

I am getting this error in the console:

ReferenceError: callback is not defined
    at getTodos (app.js:23)
    at new <anonymous> (app.js:10)
    at Object.invoke (angular.js:4473)
    at extend.instance (angular.js:9093)
    at nodeLinkFn (angular.js:8205)
    at compositeLinkFn (angular.js:7637)
    at compositeLinkFn (angular.js:7641)
    at publicLinkFn (angular.js:7512)
    at angular.js:1660
    at Scope.$eval (angular.js:15916)

Here is the my app.js:

angular.module("todoListApp", [])

.controller('mainCtrl', function($scope, dataService) {
  $scope.helloConsole = dataService.helloConsole;

  $scope.learningNgChange = function() {
    console.log("An input has changed");

  dataService.getTodos(function(response) {
    $scope.todos = response.data;

.service('dataService', function($http) {
  this.helloConsole = function() {
   console.log('This is the hello console service!'); 

  this.getTodos = function() {

Any suggestions regarding my code? Thanks,

I realised I had to define callback in the last function

this.getTodos = function(callback) {

All is working now - thanks!