数组去重

 //一种写法 使用set(map也行)

 var arr = [1,2,3,4,2,4,5,2,1,4,5,5,7];
        console.log([...new Set(arr)]);

 //二种写法--扩展运算符

 var arr2 = new Set([1,2,3,4,2,4,5,2,1,4,5,5,7]);
        [...arr2]; 

 //三种写法  Array.from() 

 var arr2 = new Set([1,2,3,4,2,4,5,2,1,4,5,5,7]);
        var arr3 = Array.from(arr2);