Group objects by multiple properties and merge array property

Question

In the example I use a each to go thru select fields and create an array of data, I need to group 2 properties year and cabin also the property months is an array that I would like to merge, thing is I'm having a bit of trouble getting the wanted result:

JSFIDDLE: http://jsfiddle.net/tc39xu6b/2/

Result I get:

[
  {
    "year": "2021",
    "cabin": "1",
    "months": [
      "1",
      "2"
    ]
  },
  {
    "year": "2021",
    "cabin": "1",
    "months": [
      "4"
    ]
  },
  {
    "year": "2021",
    "cabin": "2",
    "months": [
      "1",
      "2"
    ]
  },
  {
    "year": "2022",
    "cabin": "1",
    "months": [
      "1",
      "2"
    ]
  },
  {
    "year": "2022",
    "cabin": "1",
    "months": [
      "4"
    ]
  },
  {
    "year": "2022",
    "cabin": "2",
    "months": [
      "1",
      "2"
    ]
  }
]

and this is the result I need:

{
  "2021": [
      {"cabin":1, "months": ["1", "2","4"]},
      {"cabin":2, "months": ["1", "2"]}
  ],
  "2022": [
      {"cabin":1, "months": ["1", "2","4"]},
      {"cabin":2, "months": ["1", "2"]}
  ]
}

let obj = {}, then obj[year] = [{'cabin': cabin, 'months': months}]; in your loop — dale landry
– dale landry, Commented Mar 12, 2021 at 23:55

pilchard · Accepted Answer · 2021-03-13 00:45:11Z

This is a fairly standard group by situation. The snippet below uses reduce() to refactor your array, but you could place the logic directly in your function as it is: http://jsfiddle.net/83st9wgh/

const input = [{ "year": "2021", "cabin": "1", "months": ["1", "2"] }, { "year": "2021", "cabin": "1", "months": ["4"] }, { "year": "2021", "cabin": "2", "months": ["1", "2"] }, { "year": "2022", "cabin": "1", "months": ["1", "2"] }, { "year": "2022", "cabin": "1", "months": ["4"] }, { "year": "2022", "cabin": "2", "months": ["1", "2"] }];

const result = input.reduce((acc, { year, cabin, months }) => {
  const yearArray = (acc[year] ??= []);

  let cIndex = yearArray.findIndex(o => o.cabin === cabin);
  if (cIndex === -1) {
    cIndex = yearArray.push({ cabin, months: [] }) - 1;
  }

  const monthsArray = yearArray[cIndex].months;
  yearArray[cIndex].months = [...new Set(monthsArray.concat(months))];

  return acc;
}, {});

console.log(JSON.stringify(result, null, 2));

Tiago Coelho · Accepted Answer · 2021-03-13 00:00:02Z

So, you're midway to the solution.

from what you have I wrote a grouping function:

const addUnique=(arr1,arr2)=>{
    arr2.forEach(item=>arr1.includes(item) || arr1.push(item))
    return arr1;
}

const grouped= obj.reduce((groups,item)=>{
  const yearItems=(groups[item.year]||[])
  const cabinItem=yearItems.find(({cabin})=>cabin===item.cabin)
  const newCabinItem=cabinItem||{cabin:item.cabin};
  newCabinItem.months=addUnique(newCabinItem.months||[],item.months);
    
  return {
    ...groups,
    [item.year]: cabinItem 
        ? yearItems.map(yearItem =>
          yearItem.cabin === item.cabin 
            ? newCabinItem 
            : yearItem)
        : yearItems.concat([newCabinItem])
    }
},{})

you can see it working here: http://jsfiddle.net/9huwkz5L/

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