Merge nested array and sum the values

Question

I have data in following format,

const data = [{
        key:'key1',
        rows:[
          ["A","2018","B","1"],
          ["A","2018","C","5"],
          ["A","2018","B","4"],
          ["A","2018","C","2"],
        ]
      },
      {
        key:'key1',
        rows:[
          ["A","2018","B","10"],
          ["A","2018","C","50"],
          ["A","2018","B","40"],
          ["A","2018","C","20"],
        ]
      }]

I want to first reduce the rows and sum the last value in each array based on 3rd value. Output should be this,

const data = [{
        key:'key1',
        rows:[
          ["A","2018","B","5"],
          ["A","2018","C","7"]
        ]
      },
      {
        key:'key1',
        rows:[
          ["A","2018","B","50"],
          ["A","2018","C","70"]
        ]
      }]

After this, I want to merge these 2 object and sum the value from rows. The final output should be this,

const data = [{
        key:'key1',
        rows:[
          ["A","2018","B","55"],
          ["A","2018","C","77"]
        ]
      }]

I have tried this, but not getting desired output,

data.map(data => ({
          ...data,
          rows: data.rows.reduce((array, value) => {
            if(array.length > 0){
              let found = false;
              array.map(m => {
                if(m[2] === value[2]){
                  found = true;
                  return [...m, m[3] = parseInt(m[3]) + parseInt(value[3])]
                }
              })
              if(!found){
                array.push(value)
              }
              return array
            }else{
              array.push(value)
              return array
            }
          }, [])
      }))

Answer 1

One way of doing this is to iterate over the objects to grab the rows and flatten them, then reduce over the arrays to aggregate them, finally using Object.values to get the correct dataset at the end.

const data = [{ key: 'key1', rows: [ ["A", "2018", "B", "1"], ["A", "2018", "C", "5"], ["A", "2018", "B", "4"], ["A", "2018", "C", "2"] ] }, { key: 'key1', rows: [ ["A", "2018", "B", "10"], ["A", "2018", "C", "50"], ["A", "2018", "B", "40"], ["A", "2018", "C", "20"]]}];

const key = data[0].key;

// Use `flatMap` to iterate over the object and merge the rows
const merged = data.flatMap(el => el.rows);

const rows = Object.values(merged.reduce((acc, c) => {
  const [ k1, k2, k3, k4 ] = c;
  acc[k3] = acc[k3] || [ k1, k2, k3, 0 ];
  acc[k3][3] += +k4;
  return acc;
}, {}));

console.log([{ key, rows }]);

Note: if you need the aggregated number returned to a string simply map over the result:

.map(([ a, b, c, d ]) => {
  return [ a, b, c, d.toString() ];
});

Further reading

• flatMap
• reduce
• Object.values