Check If Some Elements of Array Are Equal in JavaScript

We have an array of numbers that have got some redundant entries, our job is to write a function that takes in the array and groups all the identical entries into one subarray and returns the new array thus formed.

For example −

//If the input array is:
const arr = [1, 3, 3, 1];
//then the output should be:
const output = [[1, 1], [3, 3]];

We will use a HashMap to keep a track of the elements already occurred and iterate over the array using a for loop, the code for this will be −

Example

const arr = [1, 3, 3, 1];
const groupArray = arr => {
   const map = {};
   const group = [];
   for(let i = 0; i < arr.length; i++){
      if(typeof map[arr[i]] === 'number'){
         group[map[arr[i]]].push(arr[i]);
      } else {
         //the push method returns the new length of array
         //and the index of newly pushed element is length-1
         map[arr[i]] = group.push([arr[i]])-1;
      }
   };
   return group;
}
console.log(groupArray(arr));

Output

The output in the console will be −

[ [ 1, 1 ], [ 3, 3 ] ]