/**
 * Get the union of arrays with unique values by means of the Set type.
 * 
 * @param {Array}
 * @return {Array}
 */
function union () {
  return [...new Set(Array.prototype.reduce.call(arguments,
    (memo, array) => {
      memo.push(...array)
      return memo
    }, []))]
}

console.log(union([1, 2, 3], [2, 3, 4])) // => [1, 2, 3, 4]