Performant JS Filter Array Unique

unique-array.js

// benchmarked in milliseconds against an array of 7 million members
// safari 12 = 79
// chrome 67 = 115
// node 10   = 97

arrFilter = (arr) => {
  return arr.filter((elem, pos, array) => {
    // if the element's index in array doesn't equal pos, it will be added
    return array.indexOf(elem) == pos
  })
}

arrDupes = (arr) => {
  // returns just the duplicated elements
  return arr.filter((elem, pos) => array.indexOf(elem) !== pos)
}