filter() 过滤方法

 var a = [1,2,3,4,2,4,5,2,1,4,5,5,7].filter(function(item,index){
            return item >3
        }); 

 //获取交集

         var arr1 = new Set([1,2,3,4,2,4,5,2,1,4,5,5,7]);
        var arr2 = new Set([9,5,2,4,8]);
        var arr3 = new Set([...arr1].filter(item => arr2.has(item)));  // 2 4 5 

 //并集 

     var arr4 = new Set([...arr1,...arr2]);

 //差集 

         var arr1 = new Set([1,2,3,4,2,4,5,2,1,4,5,5,7]);
        var arr2 = new Set([9,5,2,4,8]);
        var arr3 = new Set([...arr1].filter(item => !arr2.has(item)));  //1 3 7
        var arr3 = new Set([...arr2].filter(item => !arr1.has(item)));  //9 8