Group a JavaScript Array

Suppose, we have a JavaScript array like this −

const data = [
   {
      "dataId": "1",
      "tableName": "table1",
      "column": "firstHeader",
      "rows": [
         "a","b","c"
      ]
   },
   {
      "dataId": "2",
      "tableName": "table1",
      "column": "secondHeader",
      "rows": [
         "d","e","f",
      ]
   }, {
      "dataId": "3",
      "tableName": "table2",
      "column": "aNewFirstHeader",
      "rows": [
         1,2,3
      ]
   }
];

We are required to write a JavaScript function that in one such array. The function should construct a new grouped array of objects, based on the original array.

The grouped array should contain the data for separate, unique "tableName" objects into their own objects.

Therefore ultimately, the output should look like −

const output = [
   {
      "tableName": "table1",
      "column": ["firstHeader", "secondHeader"],
      "rows": [["a","b","c"], ["d","e","f"]]
   },
   {
      "tableName": "table2",
      "column": ["aNewFirstHeader"],
      "rows": [[1,2,3]]
   }
];

Example

const arr = [
   {
      "dataId": "1",
      "tableName": "table1",
      "column": "firstHeader",
      "rows": [ "val","b","c" ]
   },
   {
      "dataId": "2",
      "tableName": "table1",
      "column": "secondHeader",
      "rows": [
         "d","e","f",
      ]
   },
   {
      "dataId": "3",
      "tableName": "table2",
      "column": "aNewFirstHeader",
      "rows": [
         1,2,3
      ]
   }
];
const groupArray = (arr = []) => {
   const res = arr.reduce((obj => (acc, val) => {
      let item = obj.get(val.tableName);
      if (!item) {
         item = {
            tableName: val.tableName, column: [], rows: []
         }
         obj.set(val.tableName, item); acc.push(item);
      };
      item.column.push(val.column); item.rows.push(val.rows);
      return acc;
   })(new Map), []);
   return res;
};
console.log(JSON.stringify(groupArray(arr), undefined, 4));

Output

And the output in the console will be −

[
   {
      "tableName": "table1",
      "column":
      [
         "firstHeader",
         "secondHeader"
      ],
      "rows": [
         [
            "val", "b", "c"
         ],
         [
            "d", "e", "f"
         ]
      ]
   }, {
      "tableName": "table2",
      "column": [
            "aNewFirstHeader"
         ],
         "rows": [
         [
            1, 2, 3
         ]
      ]
   }
]