Reducer函数无法在Reducer对象中创建键值对

Question

I call a RSS feed and parse it using a parser. I receive an array. I now want to create an object in the end, that looks like this:

{
  "2019-06-13": {
    "rates": {
      "usd": "1.1289",
      "jpy": "122.44",
      "gbp": "0.88948",
      "chf": "1.1207"
    },
    "date": "2019-06-13"
  }
}

The .reduce function looks like this at the moment. I receive an error when I try to set the date:

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;
                }, {});

The error message: TypeError: Cannot set property 'date' of undefined

The console.log() statement gives me four values that are not undefined, ie the correct and expected values.

Answer 1

You need to create an object first before you can assign values to it:

acc[currDate] = acc[currDate] || {};
acc[currDate].date = currDate;
acc[currDate].rates = acc[currDate].rates || {};
acc[currDate].rates[currCurrency] = currRate;

The obj = obj || {} obj = obj || {} pattern is just shorthand for "create and assign a new object if it doesn't exist yet".