JavaScript Reducer函数中对象字段出现Undefined问题排查

我正在学习扩展运算符（spread operators）、reducer函数及通用数据处理方法。借助Stack Overflow的帮助，我实现了一个可按日期筛选出带ID的最新评估数据的函数，原本运行正常。但我为评估对象新增cadence和assessmentName字段后，结果中这两个字段显示为Undefined，请问问题出在哪里？

代码示例 #

const result = data.map(({assessments, ...rest}) => ({
  ...rest,
  'assessments' : Object.entries(assessments
    .reduce((a, [{ id, date }]) => {
      a[id] = (a[id] || '').localeCompare(date) > 0 ? a[id] : date
      return a
    }, {}))
    .map(([id, date, cadence, assessmentName]) => ([{ id, date, cadence, assessmentName }]))
}))

console.log(result)

数据示例 #

const data = [
  {
    "partnerId": "p0001",
    "partnerName": "Foo",
    "assessments": [
        [ { "id": "a0016", "date": "2021-03-20", "cadence": "Annual", "assessmentName": "FFIEC CAT Survey" } ],
        [ { "id": "a0016", "date": "2021-11-19", "cadence": "Annual", "assessmentName": "FFIEC CAT Survey"  } ],
        [ { "id": "a0011", "date": "2022-05-19", "cadence": "Annual", "assessmentName": "FCC Risk Assessment"  } ],
        [ { "id": "a0011", "date": "2020-10-29", "cadence": "Annual", "assessmentName": "FCC Risk Assessment"  } ],
        [ { "id": "a0001", "date": "2021-11-30", "cadence": "Semi-Annual", "assessmentName": "Counterparty & Financial Risk Assessments"  } ],
        [ { "id": "a0001", "date": "2022-06-07", "cadence": "Semi-Annual", "assessmentName": "Counterparty & Financial Risk Assessments"  } ],
        [ { "id": "a0013", "date": "2022-01-20", "cadence": "Annual", "assessmentName": "FL Risk Assessment"  } ]
    ]
  },
  {
    "partnerId": "p0002",
    "partnerName": "Bar",
    "assessments": [
        [ { "id": "a004", "date": "2021-03-20", "cadence": "Semi-Annual", "assessmentName": "Model Governance" } ],
        [ { "id": "a003", "date": "2021-11-19", "cadence": "Quarterly", "assessmentName": "Credit Strategy"  } ],
        [ { "id": "a003", "date": "2022-05-19", "cadence": "Quarterly", "assessmentName": "Credit Strategy"  } ],
        [ { "id": "a003", "date": "2020-10-29", "cadence": "Quarterly", "assessmentName": "Credit Strategy"  } ],
        [ { "id": "a0001", "date": "2021-11-30", "cadence": "Semi-Annual", "assessmentName": "Counterparty & Financial Risk Assessments"  } ],
        [ { "id": "a0001", "date": "2022-06-07", "cadence": "Semi-Annual", "assessmentName": "Counterparty & Financial Risk Assessments"  } ],
        [ { "id": "a0013", "date": "2022-01-20", "cadence": "Annual", "assessmentName": "FL Risk Assessment"  } ]
    ]
  }
]

你的reduce阶段只存储了date字符串，没有保留cadence和assessmentName这些额外字段。原来的逻辑里，a[id]仅存储日期值，后续Object.entries只能拿到id和date两个元素，所以解构cadence和assessmentName时自然会得到undefined。

需要在reduce时存储完整的评估对象，而非仅保留日期。通过对比对象中的date字段来保留最新数据，这样就能完整保留所有需要的字段：

const result = data.map(({ assessments, ...rest }) => ({
  ...rest,
  assessments: Object.values(assessments
    .reduce((acc, [assessment]) => {
      // 对比当前存储对象与新对象的日期，保留最新的那个
      if (!acc[assessment.id] || new Date(assessment.date) > new Date(acc[assessment.id].date)) {
        acc[assessment.id] = assessment;
      }
      return acc;
    }, {}))
    // 保持原数据的嵌套数组结构（每个评估包裹在数组中）
    .map(assessment => [assessment])
}));

console.log(result);

修复说明 #

• reduce中接收完整的assessment对象，不再仅解构id和date
• 使用new Date()对比日期，比localeCompare更直观可靠（ISO格式日期字符串也可直接用字符串比较，但Date对象逻辑更清晰）
• 用Object.values直接获取最新评估对象数组，比Object.entries再解构更简洁
• 最后通过map将每个对象包裹进数组，保持原数据的嵌套结构

内容的提问来源于stack exchange，提问作者mediaguru