30-seconds-of-code/snippets/filterNonUniqueBy.md

filterNonUniqueBy

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

Use Array.filter() and Array.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)));

filterNonUniqueBy(
  [
    { id: 0, value: 'a' },
    { id: 1, value: 'b' },
    { id: 2, value: 'c' },
    { id: 1, value: 'd' },
    { id: 0, value: 'e' }
  ],
  (a, b) => a.id == b.id
); // [ { id: 2, value: 'c' } ]