How to distinct and merge objects from an array of objects in Javascript

Question

I have an array of objects.

[
  { _id: { year: 2020, month: 1 } },
  { _id: { year: 2019, month: 1 } },
  { _id: { year: 2019, month: 2 } },
  { _id: { year: 2019, month: 4 } },
  { _id: { year: 2019, month: 5 } },
  { _id: { year: 2019, month: 8 } },
  { _id: { year: 2019, month: 9 } },
  { _id: { year: 2019, month: 11 } }
]

I want to remove the duplicate year properties from the objects and push different month data in one array object.

I want to get below output:

[
  {
    "_id": {
      "year": 2020,
      "month": [1]
    }
  },
  {
    "_id": {
      "year": 2019,
      "month": [1, 2, 4, 5, 8, 9, 11]
    }
  }
]

I have tried but cannot get the expected output:

let arr = [
  { _id: { year: 2020, month: 1 } },
  { _id: { year: 2019, month: 1 } },
  { _id: { year: 2019, month: 2 } },
  { _id: { year: 2019, month: 4 } },
  { _id: { year: 2019, month: 5 } },
  { _id: { year: 2019, month: 8 } },
  { _id: { year: 2019, month: 9 } },
  { _id: { year: 2019, month: 11 } }
];

let newArr= []; 
    arr.forEach(function (item) { 
        let isMatch = newArr.filter(function (elem) {
            return elem._id.year === item._id.year 
        })
        if (isMatch.length == 0) {
            newArr.push(item)
        }
        else {
            newArr.find(x => x._id.year === item._id.year)._id.month.push(...item._id.month);
        }
    })
    console.log(newArr);

mickl · Accepted Answer · 2020-03-09 04:52:23Z

1

You can try using Array.reduce:

let input = [
  { _id: { year: 2020, month: 1 } },
  { _id: { year: 2019, month: 1 } },
  { _id: { year: 2019, month: 2 } },
  { _id: { year: 2019, month: 4 } },
  { _id: { year: 2019, month: 5 } },
  { _id: { year: 2019, month: 8 } },
  { _id: { year: 2019, month: 9 } },
  { _id: { year: 2019, month: 11 } }
];

let result = input.reduce((state, current) => {
   let prev = state.find(x => x._id.year === current._id.year);
   if(prev){
      prev._id.months.push(current._id.month);
   }
   else{
      state.push({_id: { year: current._id.year, months: [ current._id.month ] } });
   }
   return state;
},[]);

console.log(result);

answered Mar 9, 2020 at 4:52

mickl

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Comments

CertainPerformance · Accepted Answer · 2020-03-09 04:50:26Z

First make an object indexed by year, whose values are the months found for that year so far, and push to an array for the year (creating if necessary) on every iteration. Afterwards, transform the object of arrays into your output structure:

const input = [
  { _id: { year: 2020, month: 1 } },
  { _id: { year: 2019, month: 1 } },
  { _id: { year: 2019, month: 2 } },
  { _id: { year: 2019, month: 4 } },
  { _id: { year: 2019, month: 5 } },
  { _id: { year: 2019, month: 8 } },
  { _id: { year: 2019, month: 9 } },
  { _id: { year: 2019, month: 11 } }
];

const monthsByYear = {};
for (const { _id: { year, month } } of input) {
  if (!monthsByYear[year]) {
    monthsByYear[year] = [];
  }
  monthsByYear[year].push(month);
}
const output = Object.entries(monthsByYear).map(
  ([year, month]) => ({ _id: { year, month } })
);
console.log(output);

brk · Accepted Answer · 2020-03-09 04:57:12Z

YOu can use reduce and findIndex and check if the accumulator array contains that year, then just push the month

let data = [{
    _id: {
      year: 2020,
      month: 1
    }
  },
  {
    _id: {
      year: 2019,
      month: 1
    }
  },
  {
    _id: {
      year: 2019,
      month: 2
    }
  },
  {
    _id: {
      year: 2019,
      month: 4
    }
  },
  {
    _id: {
      year: 2019,
      month: 5
    }
  },
  {
    _id: {
      year: 2019,
      month: 8
    }
  },
  {
    _id: {
      year: 2019,
      month: 9
    }
  },
  {
    _id: {
      year: 2019,
      month: 11
    }
  }
]

let newData = data.reduce((acc, curr) => {
  let isYrPresent = acc.findIndex((item) => {
    return item._id.year === curr._id.year;
  });

  if (isYrPresent === -1) {
    acc.push({
      _id: {
        year: curr._id.year,
        month: [curr._id.month]
      }
    })
  } else {
    acc[isYrPresent]._id.month.push(curr._id.month)
  }


  return acc;
}, []);

console.log(newData)

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