在Backbone.js中,如何让Model超类默认值作为子类的默认值？

var SuperModel = Backbone.Model.extend({
  defaults: {
    name: 'John',
    age: 25,
    gender: 'male'
  }
});

var SubModel = SuperModel.extend({
  defaults: _.extend({}, SuperModel.prototype.defaults, {
    // 添加或覆盖子类的默认值
    age: 30,
    occupation: 'engineer'
  })
});

var subModel = new SubModel();
console.log(subModel.get('name')); // 输出: John
console.log(subModel.get('age')); // 输出: 30
console.log(subModel.get('gender')); // 输出: male
console.log(subModel.get('occupation')); // 输出: engineer