使用forEach映射对象数组的Object.entries返回未定义，除非使用console.log [英] Object.entries with forEach to map array of objects returns undefined except when console.log is used

问题描述

我有一个具有键值对的对象数组，我想在每个对象中找到一个特定的键并返回值.

数组的示例:

  filterCounties = [{"StateName":特拉华州"，"CountyName":肯特"，"FIPS":10001，"Eligibles_2017":33292，"Eligibles_2018":34938，"Penetration_2017":0.107，"Penetration_2018":0.123}，{"StateName":特拉华州"，"CountyName":新城堡"，"FIPS":10003，"Eligibles_2017":94030，"Eligibles_2018":98080，"Penetration_2017":0.128，"Penetration_2018":0.164}，{"StateName":特拉华州"，"CountyName":"Sussex"，"FIPS":10005，"Eligibles_2017":61964，"Eligibles_2018":65990，"Penetration_2017":0.082，渗透率_2018":0.097}，{"StateName":特拉华州"，"CountyName":待定县名称"，"FIPS":10，"Eligibles_2017":9"Eligibles_2018":0，"Penetration_2017":0，"Penetration_2018":0}] 

我正在寻找的关键是Penetration_2018-我可以使用以下代码并使用console.log输出值

  const mapTest = new Map(Object.entries(filterCounties).forEach(([[key，value])=>console.log((key，value ["Penetration_2018"])))); 

这将输出值: console.log输出

但是，如果我先分配一个变量然后记录该变量

  const mapTest = new Map(Object.entries(filterCounties).forEach(([键，值])=>(键，值["Penetration_2018"])));console.log(`这是mapTest $ {mapTest}`); 

我得到的[object Map]没有值-我认为使用箭头功能时，返回是隐式的吗?

输出图像-带有或不带有字符串串联在此处输入图像描述

我最终想要提取这些值，以将它们分配给Victory BarChart的Y轴.感谢您的任何见解/指导.

解决方案

.forEach 不返回值或数组.尝试改用 .map .另外，在这种情况下，您不需要Java语言的 Map (它不能有重复的键)，而且还要考虑到Victory BarChart似乎将 Array 作为数据(不是 Map ).

这是我建议的更改:

  const mapTest = filterCounties.map(元素=>({'Penetration_2018':element ['Penetration_2018']})) 

I have an array of objects that have key value pairs, I want to find a specific key in each object and return the value.

A sample of the Array:

filterCounties=[
  {
    "StateName": "Delaware",
    "CountyName": "Kent",
    "FIPS": 10001,
    "Eligibles_2017": 33292,
    "Eligibles_2018": 34938,
    "Penetration_2017": 0.107,
    "Penetration_2018": 0.123
  },
  {
    "StateName": "Delaware",
    "CountyName": "New Castle",
    "FIPS": 10003,
    "Eligibles_2017": 94030,
    "Eligibles_2018": 98080,
    "Penetration_2017": 0.128,
    "Penetration_2018": 0.164
  },
  {
    "StateName": "Delaware",
    "CountyName": "Sussex",
    "FIPS": 10005,
    "Eligibles_2017": 61964,
    "Eligibles_2018": 65990,
    "Penetration_2017": 0.082,
    "Penetration_2018": 0.097
  },
  {
    "StateName": "Delaware",
    "CountyName": "Pending County Designation",
    "FIPS": 10,
    "Eligibles_2017": 9,
    "Eligibles_2018": 0,
    "Penetration_2017": 0,
    "Penetration_2018": 0
  }
]

The key I am looking for is Penetration_2018 - I can use the following code and output the values using console.log

const mapTest = new Map(
      Object.entries(filterCounties).forEach(([key, value]) =>
        console.log((key, value["Penetration_2018"]))
      )
    );

This will output the values: console.log output

However if I assign to a variable and then log the variable

 const mapTest = new Map(
      Object.entries(filterCounties).forEach(
        ([key, value]) => (key, value["Penetration_2018"])
      )
    );
    console.log(`This is mapTest ${mapTest}`);

I get [object Map] with no values - I thought that when using an arrow function, return was implicit?

Image of output - with or without String concatenation enter image description here

I ultimately want to extract these values to assign them to the Y axis of a Victory BarChart. Thanks for any insight / guidance.

解决方案

.forEach does not return a value or an array. Try using .map instead. Also, you don't need a Javascript Map in this case (it can't have repeated keys), considering furthermore that Victory BarChart seems to take an Array as data (not a Map).

This is my suggested change:

const mapTest = filterCounties.map(
  element => ({'Penetration_2018': element['Penetration_2018']})
)

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