JS使用for循环查找对象属性值总返回No such value如何修复

问题根因 #

你代码里的contacts是普通对象类型，不是数组，没有内置的length属性，所以循环条件i < contacts.length等价于i < undefined，结果永远为假，for循环从头到尾都不会执行，直接走到最后的return语句返回“No such value”。

修复方案1：调整contacts为数组结构（改动最小，适配原有循环逻辑） #

把contacts从对象改成对象数组，原有循环逻辑不用改就能正常运行：

// contacts改为对象数组，获取到length属性
var contacts = [
  {
    "Name": "X",
    "Age": "48",
    "phoneNumber": "99373xxxxx",
    "Likes": ["Cricket", "spicyFood"]
  },
  {
    "Name": "Y",
    "Age": "39",
    "phoneNumber": "79788xxxxx",
    "Likes": ["comedyShows", "spicyFood"]
  },
  {
    "Name": "Z",
    "Age": "8",
    "phoneNumber": "Not Available",
    "Likes": ["funnyVideos", "Sweets"]
  }
];

function lookUp(name, prop) {
  for (var i = 0; i < contacts.length; i++) {
    if (contacts[i].Name === name) {
      return contacts[i][prop] || "No such property";
    };
  }
  return "No such value";
}
var data = lookUp("Z", "Likes");
console.log(data); // 输出 ["funnyVideos", "Sweets"]

修复方案2：保留contacts原有对象结构，修改遍历逻辑 #

如果需要保留contacts以亲属关系为key的结构，就把循环逻辑改成遍历对象的属性值即可：

var contacts = { // 原有对象结构完全不变
  "Father": {
    "Name": "X",
    "Age": "48",
    "phoneNumber": "99373xxxxx",
    "Likes": ["Cricket", "spicyFood"]
  },
  "Mother": {
    "Name": "Y",
    "Age": "39",
    "phoneNumber": "79788xxxxx",
    "Likes": ["comedyShows", "spicyFood"]
  },
  "Sister": {
    "Name": "Z",
    "Age": "8",
    "phoneNumber": "Not Available",
    "Likes": ["funnyVideos", "Sweets"]
  }
};

function lookUp(name, prop) {
  // 遍历contacts的所有属性值（也就是所有联系人对象）
  for (const contact of Object.values(contacts)) {
    if (contact.Name === name) {
      return contact[prop] || "No such property";
    }
  }
  return "No such value";
}
var data = lookUp("Z", "Likes");
console.log(data); // 输出 ["funnyVideos", "Sweets"]

内容的提问来源于stack exchange，提问作者ANM