javascript - reduce按条件分组输出错误，为什么？

Question

要求：

const people = [
  { name: 'Alice', age: 25 },
  { name: 'Bob', age: 30 },
  { name: 'Charlie', age: 35 },
  { name: 'David', age: 25 },
  { name: 'Emily', age: 30 }
];
// Output: {
//   25: [{ name: 'Alice', age: 25 }, { name: 'David', age: 25 }],
//   30: [{ name: 'Bob', age: 30 }, { name: 'Emily', age: 30 }],
//   35: [{ name: 'Charlie', age: 35 }]
// }

代码：

let fn = (arr = []) => {
  return arr.reduce((accumulator, currentValue) => {
    return accumulator[currentValue.age]
      ? (accumulator[currentValue.age] = accumulator[currentValue.age].concat(currentValue))
      : ((accumulator[currentValue.age] = [currentValue]));
  }, {});
};

const people = [
  { name: 'Alice', age: 25 },
  { name: 'Bob', age: 30 },
  { name: 'Charlie', age: 35 },
  { name: 'David', age: 25 },
  { name: 'Emily', age: 30 }
];

console.log(fn(people));

输出：
[ { name: 'Emily', age: 30 } ]

Answer 1

你的错误在于三元表达式返回的值是数组，而你把这个数组再返回了，导致下一次接受的 accumulator 是数组而不是对象，原对象的信息实际上已经丢失。

function reduceByAge(list = []){
  return list.reduce(( reduced, presentItem ) => {
    // 获取当前条目的年龄信息
    const { age } = presentItem;

    // 已经有对应年龄的列表，则获取并命名为 subList
    // 没有的话，就给 subList 赋值空数组
    const { [age]: subList = [] } = reduced;

    // 添加当前条目到数组
    subList.push(presentItem);

    // 添加到已归并的对象中去
    reduced[age] = subList;
    return reduced;
  }, {});
}