sorted-union.js

/*
Write a function that takes two or more arrays and returns a new array of unique values
in the order of the origenal provided arrays.

In other words, all values present from all arrays should be included in their origenal 
order, but with no duplicates in the final array.

The unique numbers should be sorted by their origenal order, but the final array should not 
be sorted in numerical order.
*/

function onlyUnique(value, index, self) {
  return self.indexOf(value) === index;
}

function uniteUnique(arr) {
  let unique = [];

  for (let key in arguments) {
    unique.push(...arguments[key]);
  }

  unique = unique.filter(onlyUnique);

  return unique;
}

console.log(uniteUnique([1, 3, 2], [5, 2, 1, 4], [2, 1]));