Splitting an array based on its first value - JavaScript

Suppose we have an array of arrays of numbers like this −

const arr = [[1, 45], [1, 34], [1, 49], [2, 34], [4, 78], [2, 67], [4, 65]];

Each subarray is bound to contain strictly two elements. We are required to write a function that constructs a new array where all second elements of the subarrays that have similar first value are grouped together.

Therefore, for the array above, the output should look like −

const output = [
   [45, 34, 49],
   [34, 67],
   [78, 65]
];

We can make use of the Array.prototype.reduce() method that takes help of a Map() to construct the required array.

Example

Following is the code −

const arr = [[1, 45], [1, 34], [1, 49], [2, 34], [4, 78], [2, 67], [4, 65]];
const constructSimilarArray = (arr = []) => {
   const creds = arr.reduce((acc, val) => {
      const { map, res } = acc;
      if(!map.has(val[0])){
         map.set(val[0], res.push([val[1]]) - 1);
      }else{
         res[map.get(val[0])].push(val[1]);
      };
      return { map, res };
   }, {
      map: new Map(),
      res: []
   });
   return creds.res;
};
console.log(constructSimilarArray(arr));

Output

This will produce the following output in console −

[ [ 45, 34, 49 ], [ 34, 67 ], [ 78, 65 ] ]