使用.reduce函数进行某些计算并返回带有数组结果的对象

Question

所以我成功地获得了我追求的结果：

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);

输出：

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

这正是我所追求的，但是在处理不同的日期数组时，遵循相同的对象数组结构。我在pastDay上收到一个未定义的错误，任何人都可以帮我发现问题？或者帮助我找到.reduce

的替代方案

我遇到问题的阵列：

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);

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

第二个数组的.reduce代码：

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

第一个缩减器返回与输入数组匹配的对象格式，如

return {
    stats: nextStats,
};

你的阵列看起来像：

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

因此，当您的内部循环作为数组迭代.stats时，它将正确运行。

您的第二个缩减器正在迭代具有此结构的对象：

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

但是然后返回一个与该结构不匹配的对象：

return {
    stats: nextStats,
};

因此，reducer的第一次迭代将起作用，然后第二次迭代将运行，第一个参数pastDay将是前一次运行的返回值，它不会有任何你正在寻找的钥匙。

快速而肮脏的解决方案是在返回时匹配对象键深度：

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 } },
    };
});

虽然这回答了问题，但您使用的逻辑难以理解。根据您尝试完成的内容（代码中不清楚），这可能不是理想的方法。