使用.reduce函数进行某些计算并返回带有数组结果的对象

Question

So I've successfully managed to get the results I'm after:

 const days = [ { date: '2016-12-13T00:00:00.000Z', stats: [ { name: 'Soft Drinks', sold: 34, }, { name: 'Snacks', sold: 3, }, { name: 'Coffee and warm drinks', sold: 26, }, ], }, { date: '2016-12-14T00:00:00.000Z', stats: [ { name: 'Soft Drinks', sold: 34, }, { name: 'Snacks', sold: 3, }, { name: 'Coffee and warm drinks', sold: 26, }, ], }, ]; const newStats = days.reduce(function (pastDay, currentDay) { const nextStats = currentDay.stats.map(function(stat) { const oldSold = pastDay.stats.find(function (old) { return old.name === stat.name; }); const newSold = stat.sold + oldSold.sold; stat.sold = newSold; return stat; }); return { stats: nextStats, }; }); console.log(newStats); 

Outputs:

{
  "stats": [
    {
      "name": "Soft Drinks",
      "sold": 68
    },
    {
      "name": "Snacks",
      "sold": 6
    },
    {
      "name": "Coffee and warm drinks",
      "sold": 52
    }
  ]
}

Which is exactly what I'm after, however when dealing with a different days array, that follow the same structure of array of objects. I'm getting an undefined error on pastDay anyone can help me spot the problem? or help me find alternative to .reduce

The array I'm having trouble with:

 const days = [ { "_id":{ "_str":"f23f02994ab992437e423e24" }, "date":"2016-12-13T00:00:00.000Z", "statistics":{ "breakdown":{ "byTurnover":[ { "name":"Soft Drinks", "sold":34, "percentage":31.14 }, { "name":"Snacks", "sold":3, "percentage":2.65 }, { "name":"Coffee and warm drinks", "sold":26, "percentage":21.54 }, { "name":"Brandy", "sold":2, "percentage":2.75 }, { "name":"Beer", "sold":20, "percentage":20.15 }, { "name":"Mixed drinks Other", "sold":21, "percentage":21.77 } ], } }, "id":{ "_str":"f23f02994ab992437e423e24" } }, { "_id":{ "_str":"b3d0ad7f314e33021739f70c" }, "date":"2016-12-14T00:00:00.000Z", "statistics":{ "breakdown":{ "byTurnover":[ { "name":"Soft Drinks", "sold":34, "percentage":31.14 }, { "name":"Snacks", "sold":3, "percentage":2.65 }, { "name":"Coffee and warm drinks", "sold":26, "percentage":21.54 }, { "name":"Brandy", "sold":2, "percentage":2.75 }, { "name":"Beer", "sold":20, "percentage":20.15 }, { "name":"Mixed drinks Other", "sold":21, "percentage":21.77 } ], } }, "id":{ "_str":"b3d0ad7f314e33021739f70c" } }, { "_id":{ "_str":"e1906ce07ab811c74528e3cc" }, "date":"2016-12-15T00:00:00.000Z", "statistics":{ "breakdown":{ "byTurnover":[ { "name":"Soft Drinks", "sold":34, "percentage":31.14 }, { "name":"Snacks", "sold":3, "percentage":2.65 }, { "name":"Coffee and warm drinks", "sold":26, "percentage":21.54 }, { "name":"Brandy", "sold":2, "percentage":2.75 }, { "name":"Beer", "sold":20, "percentage":20.15 }, { "name":"Mixed drinks Other", "sold":21, "percentage":21.77 } ], } }, "id":{ "_str":"e1906ce07ab811c74528e3cc" } }, ]; const newStats = days.reduce(function (pastDay, currentDay) { const nextStats = currentDay.statistics.breakdown.byTurnover.map(function(stat) { const oldSold = pastDay.statistics.breakdown.byTurnover.find(function (old) { return old.name === stat.name; }); const newSold = stat.sold + oldSold.sold; stat.sold = newSold; return stat; }); return { stats: nextStats, }; }); console.log(newStats); 

Outputs: Uncaught TypeError: Cannot read property 'breakdown' of undefined

The .reduce code for the second array:

const newStats = days.reduce(function (pastDay, currentDay) {
  const nextStats = currentDay.statistics.breakdown.byTurnover.map(function(stat) {
    const oldSold = pastDay.statistics.breakdown.byTurnover.find(function (old) {
        return old.name === stat.name;
    });

    const newSold = stat.sold + oldSold.sold;
    stat.sold = newSold;
    return stat;
  });

  return {
    stats: nextStats,
  };
});

console.log(newStats);

Answer 1

Your first reducer is returning an object format that matches the input array, as in

return {
    stats: nextStats,
};

And your array looks like:

const days = [{ stats: [...] }]

So when your inner loop iterates over .stats as an array, it will correctly run.

Your second reducer is iterating over an object with this structure:

const days = [{ statistics: { breakdown: { byTurnover: [...] } }]

But then returns an object not matching that structure:

return {
    stats: nextStats,
};

So the first iteration of the reducer will work, then the second iteration will run, and the first argument, pastDay , will be the return value of the previous run, which won't have any of the keys you're looking up.

A quick and dirty solution is just to match the object key depth when returning:

const newStats = days.reduce(function (pastDay, currentDay) {
    const nextStats = currentDay.statistics.breakdown.byTurnover.map(function(stat) {
        const oldSold = pastDay.statistics.breakdown.byTurnover.find(function (old) {
            return old.name === stat.name;
        });

        const newSold = stat.sold + oldSold.sold;
        stat.sold = newSold;
        return stat;
    });

    return {
        statistics: { breakdown: { byTurnover: nextStats } },
    };
});

While this answers the question, the logic you're using is hard to follow. Depending on what you're trying to accomplish, which is unclear from the code, this may not be an ideal way to do it.