今天遇到的 node.js 的笔试题，我在解这道题的时候，有个问题我不太明白，感觉自己傻 X 了

function checkLegalStr(str=''){
  return (str !== '' && str !== '-' && str !== 'N/A')
}

function handleObject(obj={}){
  for(let key in obj){
    if(typeof obj[key] === 'object'){
      handleObject(obj[key]);
    }else if(Array.isArray(obj[key])){
      obj[key] = obj[key].filter(item=>{return checkLegalStr(item);});
    }else{
      if(!checkLegalStr(obj[key])){
        delete obj[key];
      }
    }
  }
}

const obj = {
  name: { first: 'Robert', middle: '', last: 'Smith' },
  age: 25,
  DOB: '-',
  hobbies: [ 'running', 'coding', '-' ],
  education: { highschool: 'N/A', college: 'Yale' }
};

console.log(obj);
handleObject(obj);
console.log(obj);

为啥这个结果 obj.hobbies 会有一个空值<1 empty item>

以下是前后输出

{
  name: { first: 'Robert', middle: '', last: 'Smith' },
  age: 25,
  DOB: '-',
  hobbies: [ 'running', 'coding', '-' ],
  education: { highschool: 'N/A', college: 'Yale' }
}
{
  name: { first: 'Robert', last: 'Smith' },
  age: 25,
  hobbies: [ 'running', 'coding', <1 empty item> ],
  education: { college: 'Yale' }
}

实在是奇怪，我感觉我好水啊，有知道的大哥吗