LazyLoad directive for loading external javascript for AngularJs. In this example, I use google maps' API as the external library being loaded

LazyLoad.js

angular.module('testApp', []).
    directive('lazyLoad', ['$window', '$q', function ($window, $q) {
        function load_script() {
            var s = document.createElement('script'); // use global document since Angular's $document is weak
            s.src = 'https://maps.googleapis.com/maps/api/js?sensor=false&callback=initialize';
            document.body.appendChild(s);
        }
        function lazyLoadApi(key) {
            var deferred = $q.defer();
            $window.initialize = function () {
                deferred.resolve();
            };
            // thanks to Emil Stenström: http://friendlybit.com/js/lazy-loading-asyncronous-javascript/
            if ($window.attachEvent) {  
                $window.attachEvent('onload', load_script); 
            } else {
                $window.addEventListener('load', load_script, false);
            }
            return deferred.promise;
        }
        return {
            restrict: 'E',
            link: function (scope, element, attrs) { // function content is optional
            // in this example, it shows how and when the promises are resolved
                if ($window.google && $window.google.maps) {
                    console.log('gmaps already loaded');
                } else {
                    lazyLoadApi().then(function () {
                        console.log('promise resolved');
                        if ($window.google && $window.google.maps) {
                            console.log('gmaps loaded');
                        } else {
                            console.log('gmaps not loaded');
                        }
                    }, function () {
                        console.log('promise rejected');
                    });
                }
            }
        };
    }]);

Good job!
Couple things to think about though:

• I would also remove load event listener once its not needed anymore:

$window.addEventListener('load', function onloadFunc () { 
    $window.removeEventListener('load', onloadFunc, false);
    load_script();
}, false);

• The event listener is not always helpful since document can be already in the "ready" state at the moment you attach the listener. Bacisally you need to check if document is already ready first and if not then attach listener. Otherwise just call load_script function.

Sorry, i dont know if i get it but in my code function load_script() is never been reach!