Group javascript array object by same key

Question

I have an array of object some with same month name but different values based on day. How do we group array object based on same property value for example:

 [ { month: 'Jan', value: 3 },
  { month: 'Jan', value: 3.5 },
  { month: 'Feb', value: 2.1 },
  { month: 'Mar', value: 6 },
  { month: 'Apr', value: 4.3 },
  { month: 'May', value: 5.5 },
  { month: 'Jun', value: 7 },
  { month: 'Jun', value: 9 },
  { month: 'Jul', value: 7 },
  { month: 'Jul', value: 9 },
  { month: 'Jul', value: 7 },
  { month: 'Aug', value: 9 },
  { month: 'Sep', value: 9 },
  { month: 'Sep', value: 9 },
  { month: 'Oct', value: 8 },
  { month: 'Oct', value: 5 },
  { month: 'Oct', value: 3 },
  { month: 'Nov', value: 12 },
  { month: 'Nov', value: 19.5 } ]

So what i want to get is new array with month and sum of month values:

[{month:"Jan", val:6.5},{month:"Feb", val:5.6},{month:"Mar", val:6},].

Possible duplicate of Most efficient method to groupby on a array of objects — H77
– H77, Commented Nov 23, 2018 at 10:03

Nina Scholz · Accepted Answer · 2018-11-23 10:07:15Z

You could take a Map and reduce the array by adding the value to the month nad get new arrays from the grouped result.

var data = [{ month: 'Jan', value: 3 }, { month: 'Jan', value: 3.5 }, { month: 'Feb', value: 2.1 }, { month: 'Mar', value: 6 }, { month: 'Apr', value: 4.3 }, { month: 'May', value: 5.5 }, { month: 'Jun', value: 7 }, { month: 'Jun', value: 9 }, { month: 'Jul', value: 7 }, { month: 'Jul', value: 9 }, { month: 'Jul', value: 7 }, { month: 'Aug', value: 9 }, { month: 'Sep', value: 9 }, { month: 'Sep', value: 9 }, { month: 'Oct', value: 8 }, { month: 'Oct', value: 5 }, { month: 'Oct', value: 3 }, { month: 'Nov', value: 12 }, { month: 'Nov', value: 19.5 }],
    result = Array.from(
        data.reduce(
            (m, { month, value }) => m.set(month, (m.get(month) || 0) + value),
            new Map
        ),
        ([month, value]) => ({ month, value })
    );

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Tanmoy Krishna Das · Accepted Answer · 2018-11-23 10:14:14Z

A simple human-readable solution can be as follows:

var input = [ { month: 'Jan', value: 3 },
  { month: 'Jan', value: 3.5 },
  { month: 'Feb', value: 2.1 },
  { month: 'Mar', value: 6 },
  { month: 'Apr', value: 4.3 },
  { month: 'May', value: 5.5 },
  { month: 'Jun', value: 7 },
  { month: 'Jun', value: 9 },
  { month: 'Jul', value: 7 },
  { month: 'Jul', value: 9 },
  { month: 'Jul', value: 7 },
  { month: 'Aug', value: 9 },
  { month: 'Sep', value: 9 },
  { month: 'Sep', value: 9 },
  { month: 'Oct', value: 8 },
  { month: 'Oct', value: 5 },
  { month: 'Oct', value: 3 },
  { month: 'Nov', value: 12 },
  { month: 'Nov', value: 19.5 } ];

var result = [];

for (var i = 0; i < input.length; i++) {
    var data = input[i];
    var found=false;
    for(var j=0; j<result.length; j++) {
        if(result[j].month === data.month) {
            found=true;
            result[j].value += data.value;
            break;
        }
    }
    if(!found) {
        result.push(data);
    }
}

//The result array is your desired result
console.log(result);

Endar · Accepted Answer · 2018-11-23 10:49:55Z

0

 result = input.reduce(function(res, value){
            if (!res[value.month]) {
                res[value.month] = {
                    month: value.month,
                    value: 0
                }
                result.push(res[value.month])
            }
            res[value.month].value += value.value
            return res;
           }, {});
             console.log(result)

answered Nov 23, 2018 at 10:49

Endar

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