Reducer 函数无法在 reducer 对象中创建键值对

Question

我调用 RSS 提要并使用解析器对其进行解析。我收到一个数组。我现在想最终创建一个对象，如下所示：

{

  "2019-06-13": {

    "rates": {

      "usd": "1.1289",

      "jpy": "122.44",

      "gbp": "0.88948",

      "chf": "1.1207"

    },

    "date": "2019-06-13"

  }

}

该.reduce功能目前看起来像这样。当我尝试设置日期时收到错误消息：

rateJson = rateArray.reduce((acc, curr) => {

                    let currObj =

                        curr['cb:statistics'][0]['cb:exchangeRate'][0];

                    let currCurrency = currObj['cb:targetCurrency'][0].toLowerCase();

                    let currDate =

                        currObj['cb:observationPeriod'][0]['cb:period'][0];

                    let currRate = {

                        rate: currObj['cb:observation'][0]['cb:value'][0],

                        mult: currObj['cb:observation'][0]['cb:unit_mult'][0]

                    };

                    console.log(currDate,currCurrency,currRate, acc)

                    acc[currDate]['date'] = currDate;

                    acc[currDate]['rates'][currCurrency] = currRate;

                    return acc;

                }, {});

错误信息： TypeError: Cannot set property 'date' of undefined

该console.log()语句给了我四个未定义的值，即正确的和预期的值。

Answer 1

您需要先创建一个对象，然后才能为其赋值：acc[currDate] = acc[currDate] || {};acc[currDate].date = currDate;acc[currDate].rates = acc[currDate].rates || {};acc[currDate].rates[currCurrency] = currRate;该obj = obj || {}模式只是“如果尚不存在则创建并分配新对象”的简写。