JavaScript遍历嵌套JSON对象 映射各用户skills数组长度

for...in 遍历普通对象时，循环变量返回的永远是对象的字符串类型键名，而非键对应的属性值，这是JavaScript的固定语法规则，和JSON.parse的调用无关。你传入JSON.parse的第二个参数undefined等价于不传参，解析得到的userObj本身没有问题。

你需要的「用户名对应用户skills数组长度」的映射，有两种可直接运行的写法：

• 写法1：修正原有for...in逻辑，拿到键名后主动取对应的属性值

const txt = `{
    "Alex": {
        "email": "alex@alex.com",
        "skills": [
            "HTML",
            "CSS",
            "JavaScript"
        ],
        "age": 20,
        "isLoggedIn": false,
        "points": 30
    },
    "Asab": {
        "email": "asab@asab.com",
        "skills": [
            "HTML",
            "CSS",
            "JavaScript",
            "Redux",
            "MongoDB",
            "Express",
            "React",
            "Node"
        ],
        "age": 25,
        "isLoggedIn": false,
        "points": 50
    },
    "Brook": {
        "email": "daniel@daniel.com",
        "skills": [
            "HTML",
            "CSS",
            "JavaScript",
            "React",
            "Redux"
        ],
        "age": 30,
        "isLoggedIn": true,
        "points": 50
    },
    "Daniel": {
        "email": "daniel@alex.com",
        "skills": [
            "HTML",
            "CSS",
            "JavaScript",
            "Python"
        ],
        "age": 20,
        "isLoggedIn": false,
        "points": 40
    },
    "John": {
        "email": "john@john.com",
        "skills": [
            "HTML",
            "CSS",
            "JavaScript",
            "React",
            "Redux",
            "Node.js"
        ],
        "age": 20,
        "isLoggedIn": true,
        "points": 50
    },
    "Thomas": {
        "email": "thomas@thomas.com",
        "skills": [
            "HTML",
            "CSS",
            "JavaScript",
            "React"
        ],
        "age": 20,
        "isLoggedIn": false,
        "points": 40
    },
    "Paul": {
        "email": "paul@paul.com",
        "skills": [
            "HTML",
            "CSS",
            "JavaScript",
            "MongoDB",
            "Express",
            "React",
            "Node"
        ],
        "age": 20,
        "isLoggedIn": false,
        "points": 40
    }
}`;
const userObj = JSON.parse(txt);
const skillLengthMap = {};
for (const username in userObj) {
  // 用键名取到对应用户对象，再读取skills数组长度
  skillLengthMap[username] = userObj[username].skills.length;
}
console.log(skillLengthMap);

• 写法2：用Object.entries+Object.fromEntries实现，写法更简洁

const userObj = JSON.parse(txt);
const skillLengthMap = Object.fromEntries(
  Object.entries(userObj).map(([username, userInfo]) => [username, userInfo.skills.length])
);
console.log(skillLengthMap);

运行上述代码后得到的输出如下：

{
  Alex: 3,
  Asab: 8,
  Brook: 5,
  Daniel: 4,
  John: 6,
  Thomas: 4,
  Paul: 7
}

注意你给出的预期结果存在两处笔误：一是Asab的skills实际有8项不是7项；二是原数据里对应用户名是Brook不是Brooke。

内容的提问来源于stack exchange，提问作者conradQQ