javascript - 为什么我的 Javascript 代码没有向对象添加新属性？

Question

我正在学习 JavaScript 的 freeCodeCamp 教程，并且一直在完成记录集合作业。我正在尝试以不同于所呈现的方式编写它，但是尽管浏览了过去的教程并在线搜索，我无法理解为什么我的代码没有将属性“艺术家”附加到 5439 对象。

var collection = {
    "2548": {
      "album": "Slippery When Wet",
      "artist": "Bon Jovi",
      "tracks": [ 
        "Let It Rock", 
        "You Give Love a Bad Name" 
      ]
    },
    "2468": {
      "album": "1999",
      "artist": "Prince",
      "tracks": [ 
        "1999", 
        "Little Red Corvette" 
      ]
    },
    "1245": {
      "artist": "Robert Palmer",
      "tracks": [ ]
    },
    "5439": {
      "album": "ABBA Gold"
    }
};
// Keep a copy of the collection for tests
var collectionCopy = JSON.parse(JSON.stringify(collection));

// Only change code below this line
function updateRecords(id, prop, value) {
  for (var i in collection){
    if (i == id){
      if (value == ""){
        delete collection.i.prop;
      }
      else if (prop == "tracks" && value !== ""){
        if (collections.hasOwnProperty("tracks")){
          i.tracks.push(value);
        }
        else {
          i["tracks"] = value;
        }
      }
      else {
        i.prop = value;
      }
    }
  }
  return collection;
}

// Alter values below to test your code
console.log(updateRecords(5439, "artist", "ABBA"));

我浏览了在线教程，看起来我正确地包含了它。我也尝试过括号表示法，但没有任何效果，但是当我调试代码并打印结果时，没有添加任何内容。我根本就没有访问该对象吗？

Answer 1

您需要使用bracket notation当使用变量动态访问对象集合中的属性时。当您使用 for...in 循环时，您的 i 变量引用对象中的键，因此它不会具有 .tracks 等属性 等...(这意味着诸如 i.tracks.push(value) 之类的代码将不起作用)，您需要获取对存储在键 i< 处的对象的引用 使用 collection[i]:

var collection = {
    "2548": {
      "album": "Slippery When Wet",
      "artist": "Bon Jovi",
      "tracks": [ 
        "Let It Rock", 
        "You Give Love a Bad Name" 
      ]
    },
    "2468": {
      "album": "1999",
      "artist": "Prince",
      "tracks": [ 
        "1999", 
        "Little Red Corvette" 
      ]
    },
    "1245": {
      "artist": "Robert Palmer",
      "tracks": [ ]
    },
    "5439": {
      "album": "ABBA Gold"
    }
};
// Keep a copy of the collection for tests
var collectionCopy = JSON.parse(JSON.stringify(collection));

// Only change code below this line
function updateRecords(id, prop, value) {
  for (var i in collection){
    if (i == id){
      if (value == ""){
        delete collection[i][prop];
      }
      else if (prop == "tracks" && value !== ""){
        if (collections.hasOwnProperty("tracks")){
          collection[i].tracks.push(value);
        }
        else {
          collection[i].tracks = value;
        }
      }
      else {
        collection[i][prop] = value;
      }
    }
  }
  return collection;
}

// Alter values below to test your code
console.log(updateRecords(5439, "artist", "ABBA"));

但是，由于您当前所做的只是更新对象，因此在提供 id 时不需要 for...in 循环>，作为更新的关键:

var collection = {2548:{album:"Slippery When Wet",artist:"Bon Jovi",tracks:["Let It Rock","You Give Love a Bad Name"]},2468:{album:"1999",artist:"Prince",tracks:["1999","Little Red Corvette"]},1245:{artist:"Robert Palmer",tracks:[]},5439:{album:"ABBA Gold"}};
// Keep a copy of the collection for tests
var collectionCopy = JSON.parse(JSON.stringify(collection));

// Only change code below this line
function updateRecords(id, prop, value) {
  if (id in collection) {
    var to_update = collection[id];
    if (value == "") {
      delete to_update[prop];
    } else if (prop == "tracks") { // only get to this point if `value !== ""`, so there is no need to check `&& value !== ""`
      if ('tracks' in to_update)
        to_update.tracks.push(value);
      else
        to_update.tracks = value;
    } else {
      to_update[prop] = value;
    }
  }
  return collection;
}

// Alter values below to test your code
console.log(updateRecords(5439, "artist", "ABBA"));