nonUniqueElements.js

/* eslint-disable camelcase */
/*
You are given a non-empty list of integers (X).

For this task, you should return a list consisting of
only the non-unique elements in this list.

To do so you will need to remove all unique elements
(elements which are contained in a given list only once).

When solving this task, do not change the order of the list.

Example:

input (array of integers): [1, 2, 3, 1, 3]
output (iterable of integers): [1, 3, 1, 3]

1 and 3 are non-unique elements.

More examples:

nonUniqueElements([1, 2, 3, 1, 3]) == [1, 3, 1, 3]
nonUniqueElements([1, 2, 3, 4, 5]) == []
nonUniqueElements([5, 5, 5, 5, 5]) == [5, 5, 5, 5, 5]
nonUniqueElements([10, 9, 10, 10, 9, 8]) == [10, 9, 10, 10, 9]
 */

 function isNotUnique(item, index, array){
   const array_without_current_elem = array.slice()
   delete array_without_current_elem[index]
  const found = array_without_current_elem.find(a => a === item)
  if(found === undefined){
    return false
  }
    return true
 }

export function nonUniqueElements(data) {
  if(Array.isArray(data)){
    return data.filter(isNotUnique)
  }
    return false
}