Sorted Union

function uniteUnique(arr) {
  //合并数组
  //刷选唯一
  let allInOne= [...arguments].reduce((arrA, arrB)=> [...arrA, ...arrB])
console.log(`allInOne: ${allInOne}`)
  let sortedUnionArr= allInOne.reduce((array, item)=> {
console.log(array+item+array.includes(item))
    return array.includes(item)?array: [...array, item]
  }, [])
  return sortedUnionArr
}

uniteUnique([1, 3, 2], [5, 2, 1, 4], [2, 1]);

测试数据：

uniteUnique([1, 3, 2], [5, 2, 1, 4], [2, 1]) should return [1, 3, 2, 5, 4].

uniteUnique([1, 2, 3], [5, 2, 1]) should return [1, 2, 3, 5].

uniteUnique([1, 2, 3], [5, 2, 1, 4], [2, 1], [6, 7, 8]) should return [1, 2, 3, 5, 4, 6, 7, 8].

//别人家的孩子
function uniteUnique(...arrays) {//接收所有参数，放进arrays 数组中
  //make an array out of the given arrays and flatten it (using the spread operator)
	//concat 可以一口气连接多个数组。[].concat(arr1, arr2, arr3)
  const flatArray = [].concat(...arrays);

  // create a Set which clears any duplicates since it's a regulat set and not a multiset
	//new Set()返回的是对象{}，对象转换成数组[...obj]
  return [...new Set(flatArray)];
}

// test here
uniteUnique([1, 3, 2], [5, 2, 1, 4], [2, 1]);