Javascript forEach-TypeError：无法设置未定义的属性

Question

我有以下要重组的数组

    {
        "feedId": 1,
        "status": "read",
        "count": 105
    },
    {
        "feedId": 2,
        "status": "read",
        "count": 46
    },
    {
        "feedId": 3,
        "status": "read",
        "count": 29
    }

当我使用以下函数时，出现错误TypeError：无法设置未定义的属性“ read”。

    var feeds = [];

    statusCountGrouped.forEach(function(item) {
        feeds[item.feedId][item.status] = item.count;
    });

为什么不起作用？

Answer 1

feeds [item.feedId] 未初始化。最简单的解决方案是根据需要在foreach中进行初始化。我有两种解决方案，如您所愿：

1 /

const statusCountGrouped = [
  {
      "feedId": 1,
      "status": "read",
      "count": 105
  },
  {
      "feedId": 2,
      "status": "read",
      "count": 46
  },
  {
      "feedId": 3,
      "status": "read",
      "count": 29
  }
];

var feeds = [];

statusCountGrouped.forEach(function(item) {
    if(feeds[item.feedId] === undefined) { feeds[item.feedId] = {}; }
    feeds[item.feedId][item.status] = item.count;
});

console.log(feeds);

2 /

const statusCountGrouped = [
  {
      "feedId": 1,
      "status": "read",
      "count": 105
  },
  {
      "feedId": 2,
      "status": "read",
      "count": 46
  },
  {
      "feedId": 3,
      "status": "read",
      "count": 29
  }
];

var feeds = [];

statusCountGrouped.forEach(function(item) {
  feeds[item.feedId] = feeds[item.feedId] || {};
  feeds[item.feedId][item.status] = item.count;
});

console.log(feeds);

Answer 2

您收到错误消息，因为在分配之前未定义feeds[item.feedId]。您应该先初始化它，然后再为feeds[item.feedId]

赋值

var feeds = [];

i.forEach(function(item) {
    if (feeds[item.feedId] === undefined) feeds[item.feedId] = [];
    feeds[item.feedId][item.status] = item.count;
});

希望这会有所帮助。