union.js

const union = (arr1, arr2) => utility.uniq([...arr1, ...arr2])
var utility = {union, uniq}

function uniq (arr) {
	var elemMap = {}
  for(var val of arr){
    elemMap[val] = elemMap[val] ? elemMap[val] += 1 : 1
  }
  return Object.keys(elemMap).map(function (key) {
    return Number(key)
  }) 
}

console.log(utility.union([1,3,7,8], [1,3,4,5]));