Javascript - Take 2 or more arrays and return an array of unique value in the original order

union.js

function unite(arr1, arr2, arr3) {
   return Array.from(arguments).reduce(function(a, b){
  	return a.concat(b).filter(function(val,pos,arr){
      return arr.indexOf(val) === pos;
    });
  });
}

unite([1, 2, 3], [5, 2, 1, 4], [2, 1], [6, 7, 8]);