给你一个实际demo吧：

Plunker

angular.module('demo', []).service('BaiduMap', function($window, $document, $q){

  var promise;
  
  this.load = function(){
      if(promise){
        return promise;
      }
      
      promise = $q(function(resolve, reject){
        
          $window.initMap = function(){
              resolve();
              return;
          };
  
          var script = document.createElement("script");
          script.type = "text/javascript";
          script.src = "http://api.map.baidu.com/api?v=2.0&ak=E4805d16520de693a3fe707cdc962045&callback=initMap";
          $document[0].body.appendChild(script);
          
      });
      
      return promise;
  };
  
});