let users = {
  Alan: {
    age: 27,
    online: false
  },
  Jeff: {
    age: 32,
    online: true
  },
  Sarah: {
    age: 48,
    online: false
  },
  Ryan: {
    age: 19,
    online: true
  }
};

function countOnline(obj) {
  // 请把你的代码写在这条注释以下
  let count = 0
  for(let user in obj) {
    if(user.online) {
      count += 1
    }
  }
  return count
  // 请把你的代码写在这条注释以上
}
alert(countOnline(users))