Call it trickery or geekery, I am going to create a unique array using `filter` (thanks @getify)

unique_array.js

const the_redundants = [1, 2, 2, 3, 4, 6, 6, 7, 7, 7, 8]

// this is posiible because indexOf always
// returns the left most index
const the_pures = the_redundants
  .filter((n, index, arr) => 
     arr.indexOf(n) === index
   )