function sortedUnion(){
  // first put arguments into array
  return Array.from(arguments)
    // now merge all arrays into a single array through 'reduce' function
    .reduce(function(arrA,arrB){
    return arrA.concat(arrB);
  }, [])

  // now remove duplicates using 'filter' function
  .filter(function(value, index, arr){
    return arr.indexOf(value) === index;
  })
}