Get the union of arrays by means of the Set type.

union.js

/**
 * Get the union of arrays with unique values by means of the Set type.
 * 
 * @param {Array[]}
 * @return {Set}
 */
function union () {
  return new Set(arguments[0].concat(
    ...Array.prototype.slice.call(arguments, 1)))
}

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