如何使用下划线js返回自定义分组？

<script src="underscore.js"></script>

var students = [
  { name: 'Alice', age: 20, gender: 'female' },
  { name: 'Bob', age: 22, gender: 'male' },
  { name: 'Charlie', age: 21, gender: 'male' },
  { name: 'Diana', age: 20, gender: 'female' }
];

var groupedStudents = _.groupBy(students, function(student) {
  return student.age;
});

_.each(groupedStudents, function(group, age) {
  console.log('Students with age ' + age + ':');
  _.each(group, function(student) {
    console.log(student.name);
  });
});