old_contents: import Ember from 'ember';
  
  const o = Ember.observer;
  const on = Ember.on;
  
  export default Ember.ObjectProxy.extend({
    /*
     * Add method from content
     * @param method[function]
     * @return function
     */
    _addMethod(method){
      this[method] = function() {
        var content = this.get('content');
  
        if (Ember.isEmpty(content[method])) {
          return;
        }
  
        return content[method].apply(this, arguments);
      };
    },
  
    /*
     * Setup method proxying
     */
    _setupMethods: on('init', o('content', function(){
      var content = this.get('content');
  
      if (Ember.isEmpty(content)) { return; }
  
      for (var method in content) {
        if(Ember.typeOf(content[method]) === "function" && Ember.isEmpty(this[method])) {
           this._addMethod(method);
        }
      }
    })),
  
    /*
     * Setup ember data relationship keys
     */
    _setupEmberData: on('init', o('content', function() {
      var content = this.get('content');
  
      if (Ember.isEmpty(content)) { return; }
  
      this._internalModel = content._internalModel;
    }))
  });
  
new_contents: import Ember from 'ember';
  
  const o = Ember.observer;
  const on = Ember.on;
  
  export default Ember.ObjectProxy.extend({
    /*
     * Add method from content
     * @param method[function]
     * @return function
     */
    _addMethod(method){
      this[method] = function() {
        var content = this.get('content');
  
        if (Ember.isEmpty(content[method])) {
          return;
        }
  
        return content[method].apply(this, arguments);
      };
    },
  
    /*
     * Setup method proxying
     */
    _setupMethods: on('init', o('content', function(){
      var content = this.get('content');
  
      if (Ember.isEmpty(content)) { return; }
  
      for (let method in content) {
        if(Ember.typeOf(content[method]) === "function" && Ember.isEmpty(this[method])) {
           this._addMethod(method);
        }
      }
    })),
  
    /*
     * Setup ember data relationship keys
     */
    _setupEmberData: on('init', o('content', function() {
      var content = this.get('content');
  
      if (Ember.isEmpty(content)) { return; }
  
      this._internalModel = content._internalModel;
    }))
  });
  
subject: Change var to let by ES6