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Filters out the non-unique values in an array, based on a provided comparator function.

Use Array.prototype.filter() and Array.prototype.every() for an array containing only the unique values, based on the comparator function, fn. The comparator function takes four arguments: the values of the two elements being compared and their indexes.

const filterNonUniqueBy = (arr, fn) =>
  arr.filter((v, i) => arr.every((x, j) => (i === j) === fn(v, x, i, j)));