JavaScript：使用 reduce() 和 map() 简化数组导致未定义

Posted 2023-03-16

【中文标题】JavaScript：使用 reduce() 和 map() 简化数组导致未定义

【英文标题】：JavaScript : Simplifying array with reduce() and map() resulting in undefined

【发布时间】：2021-10-30 13:59:42

【问题描述】：

sample = [
    
      key1: 'value1',
      key2: 'val1',
      key3: [ subkey1: 'a', subkey2: 'b' ]
    ,
    
      key1: 'value2',
      key2: 'val2',
      key3: [ subkey1: 'c', subkey2: 'd' ]
    ,
    
      key1: 'value3',
      key2: 'val3',
      key3: [ subkey1: 'e', subkey2: 'f' ]
    ,
    
      key1: 'value3',
      key2: 'val3',
      key3: [ subkey1: 'g', subkey2: 'h' ]
    ,
    
      key1: 'value3',
      key2: 'val3',
      key3: [ subkey1: 'i', subkey2: 'j' ]
    ,
     key1: 'value3', key2: 'val3', key3: [ subkey1: 'k', subkey2: 'l' ] 
  ];

  result = Array.from(
    this.sample.reduce(
      (m,  key1, key3, key2 ) =>
        m.set(key1, [...(m.get(key1) || []), key3, key2]),
      new Map()
    ),
    ([key1, key3, key2]) => ( key1, key3, key2 )
  );
  
  console.log(result);

我是 javascript 新手，正在尝试对如下所示的对象数组执行分组操作：

sample = [key1: "value1", key2: "val1", key3: [subkey1: "a", subkey2: "b"]
key1: "value2", key2: "val2", key3: [subkey1: "c", subkey2: "d"]
key1: "value3", key2: "val3", key3: [subkey1: "e", subkey2: "f"]
key1: "value3", key2: "val3", key3: [subkey1: "g", subkey2: "h"]
key1: "value3", key2: "val3", key3: [subkey1: "i", subkey2: "j"]
key1: "value3", key2: "val3", key3: [subkey1: "k", subkey2: "l"]]

我尝试使用以下代码根据 key1 值对其进行分组：

const result = Array.from(sample.reduce((m, key1, key3, key2) =>
m.set(key1, [...(m.get(key1) || []), key3, key2]), new Map()), ([key1, key3, key2]) =>
(key1, key3, key2));

但是我得到了这样的东西：

[key1: "value3", key3: [subkey1: "e", subkey2: "f",subkey1: "g", 
subkey2: "h",subkey1: "i", subkey2: "j",subkey1: "k", subkey2: 
"l"], key2: **undefined**,.....etc]

预期：

[key1: "value3", key3: [subkey1: "e", subkey2: "f",subkey1: "g", 
subkey2: "h",subkey1: "i", subkey2: "j",subkey1: "k", subkey2: 
"l"],, key2: "val3",.....etc]

key 2 在这里仍未定义。有人可以告诉我我在哪里做错了或任何其他方式来获得预期的结果。

谢谢，

【问题讨论】：

@HR01M8055 分组基于 key1。

【参考方案1】：

您错误地使用了Array.from() 和Map 对象，基本上是多余的。 您将 4 个参数传递给 Map.set()，而它只需要 2 个参数。

let m = new Map();
m.set("1","2","3","4"); // essentially just m.set("1","2"), which set the key "1" to value "2";
let array = Array.from(m);
let array2 = Array.from(m,([a,b,c,d])=>return a,b,c,d);
console.log(array);
console.log(array2);

最重要的是您不确定要如何对对象进行分组。您的示例未提供任何信息，因为除了 key1 的值之外的所有值都是相等的。当按key1 的值分组而组成员对key2 和key3 有不同的值时，您期望什么结果？请仔细考虑您的预期输入和输出是什么。设计清晰的数据结构。