带有对象的嵌套数组，lodash meanBy

Question

有人可以帮助我了解如何进行这项工作。每次我觉得我开始理解 Javascript 中的数组和对象时，结果证明我仍然没有。

我正在尝试使用 lodash meanBy 获取以下数据结构中所有价格的平均值

[
  {
    date: "2019-12-17",
    items: [
      { id: "1", state: "accepted", price: "90.5" },
      { id: "2", state: "rejected", price: "20.0" },
      { id: "3", state: "open", price: "10.5" },
    ]
  },
  {
    date: "2019-12-18",
    items: [
      { id: "4", state: "open", price: "450.0" },
      { id: "5", state: "rejected", price: "40.1" },
      { id: "6", state: "accepted", price: "50.9" },
    ]
  }
]

如果您提供了答案，您能否也请尝试解释一下您如何选择嵌套在项目中的东西，因为这是我迷路之前所能得到的。

Answer 1

在这种情况下，与其选择嵌套值，不如将项目展平为单个数组，然后应用_.meanBy()。此外，价格是字符串，而不是数字，因此您需要对其进行转换。

用Array.flatMap()将items扁平化为一个数组，然后用_.meanBy()，得到价格的数值：

const data = [{"date":"2019-12-17","items":[{"id":"1","state":"accepted","price":"90.5"},{"id":"2","state":"rejected","price":"20.0"},{"id":"3","state":"open","price":"10.5"}]},{"date":"2019-12-18","items":[{"id":"4","state":"open","price":"450.0"},{"id":"5","state":"rejected","price":"40.1"},{"id":"6","state":"accepted","price":"50.9"}]}]

const result = _.meanBy(_.flatMap(data, 'items'), o => +o.price)

console.log(result)

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.15/lodash.js"></script>

另一种方法是获取一般平均值，通过分别获取每个items数组的平均值，然后获取所有平均值的平均值。

const data = [{"date":"2019-12-17","items":[{"id":"1","state":"accepted","price":"90.5"},{"id":"2","state":"rejected","price":"20.0"},{"id":"3","state":"open","price":"10.5"}]},{"date":"2019-12-18","items":[{"id":"4","state":"open","price":"450.0"},{"id":"5","state":"rejected","price":"40.1"},{"id":"6","state":"accepted","price":"50.9"}]}]

const result = _.meanBy(data, ({ items }) => _.meanBy(items, o => +o.price))

console.log(result)

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.15/lodash.js"></script>