Group array by equal values JavaScript

Let’s say, we have an array of string / number literals that contains some duplicate values like this −

const array = ['day', 'night', 'afternoon', 'night', 'noon', 'night', 'noon', 'day', 'afternoon', 'day', 'night'];

We are required to write a function groupSimilar() that takes in this array and returns a new array where all the repeating entries are group together in a subarray as the first element and their total count in the original array as the second element.

So, for this example, the output should be −

[
   [ 'day', 3 ],
   [ 'night', 4 ],
   [ 'afternoon', 2 ],
   [ 'noon', 2 ]
]

Let’s write the code for this function. We will use the Array.prototype.map() function to construct a new, required, array, and we will use a Map to keep track of the repeating entries in the array −

Example

const array = ['day', 'night', 'afternoon', 'night', 'noon', 'night',
'noon', 'day', 'afternoon', 'day', 'night'];
const groupSimilar = arr => {
   return arr.reduce((acc, val) => {
      const { data, map } = acc;
      const ind = map.get(val);
      if(map.has(val)){
         data[ind][1]++;
      } else {
         map.set(val, data.push([val, 1])-1);
      }
      return { data, map };
   }, {
      data: [],
      map: new Map()
   }).data;
};
console.log(groupSimilar(array));

Output

The output in the console will be −

[ [ 'day', 3 ], [ 'night', 4 ], [ 'afternoon', 2 ], [ 'noon', 2 ] ]