Find all duplicates in an array. The elements in the array have no restrictions, but in this algorithm we'll work specifically with integers.

find_duplicates.js

/*
Example:

duplicates([1, 21, -4, 103, 21, 4, 1]);
result = [1, 21]
*/

function find_duplicates(arr) {
  const duplicates = {};
    
  arr.forEach((item) => {
    if (duplicates[item]) 
      duplicates[item]++;
    } else {
      duplicates[item] = 1; 
    }
  });
    
  return Object
    .keys(duplicates)
    .filter(key => duplicates[key] > 1);
}

console.log(find_duplicates([1, 21, -4, 103, 21, 4, 1]));