///集合取交集
Array.intersect = function () {
  var result = new Array();
  var obj = {};
  for (var i = 0; i < arguments.length; i++) {
    for (var j = 0; j < arguments[i].length; j++) {
      var str = arguments[i][j];
      if (!obj[str]) {
        obj[str] = 1;
      }
      else {
        obj[str]++;
        if (obj[str] == arguments.length)
        {
          result.push(str);
        }
      }
    }
  }
  return result;
}
//集合去掉重復
Array.prototype.uniquelize = function () {
  var tmp = {},
    ret = [];
  for (var i = 0, j = this.length; i < j; i++) {
    if (!tmp[this[i]]) {
      tmp[this[i]] = 1;
      ret.push(this[i]);
    }
  }
  return ret;
}
//并集
Array.union = function () {
  var arr = new Array();
  var obj = {};
  for (var i = 0; i < arguments.length; i++) {
    for (var j = 0; j < arguments[i].length; j++)
    {
      var str=arguments[i][j];
      if (!obj[str])
      {
        obj[str] = 1;
        arr.push(str);
      }
    }//end for j
  }//end for i
  return arr;
}
//2個集合的差集 在arr不存在
Array.prototype.minus = function (arr) {
  var result = new Array();
  var obj = {};
  for (var i = 0; i < arr.length; i++) {
    obj[arr[i]] = 1;
  }
  for (var j = 0; j < this.length; j++) {
    if (!obj[this[j]])
    {
      obj[this[j]] = 1;
      result.push(this[j]);
    }
  }
  return result;
};
console.log(Array.intersect(["1", "2", "3"], ["2", "3", "4", "5", "6"]));//[2,3]
console.log([1, 2, 3, 2, 3, 4, 5, 6].uniquelize());//[1,2,3,4,5,6]
console.log(Array.union(["1", "2", "3"], ["2", "3", "4", "5", "6"], ["5", "6", "7", "8", "9"]))
console.log(["2", "3", "4", "5", "6"].minus(["1", "2", "3"]));

const arr1 = [1,2,3,4];
const arr2 = [3,4,5,6];
const arr3 = [1,2,3,2,1,3,4];
let s1 = new Set(arr1);
let s2 = new Set(arr2);
//并集
let union = [...s1, ...s2];
console.log(union); //[1, 2, 3, 4, 3, 4, 5, 6]
//交集
let intersect = arr1.filter( x => s2.has(x) );
console.log(intersect); //[3, 4]
//差集
let difference = arr1.filter( x => !s2.has(x) );
console.log(difference); //[1, 2]
//去重
let minus = [...new Set(arr3)];
console.log(minus); //[1, 2, 3, 4]