filterUnique

Creates an array with the unique values filtered out.

• Use new Set() and the spread operator (...) to create an array of the unique values in arr.
• Use Array.prototype.filter() to create an array containing only the non-unique values.

const filterUnique = arr =>
  [...new Set(arr)].filter(i => arr.indexOf(i) !== arr.lastIndexOf(i));

filterUnique([1, 2, 2, 3, 4, 4, 5]); // [2, 4]