用JS编写一个函数，返回数组中重复出现过的元素

    var arr = [1, 2, 3, 1, 2, 3, 4, 5];
    var getRepeat = function (arr) {
        var obj = {};
        for (var i = 0, len = arr.length; i < len; i++) {
            if (obj[arr[i]] == undefined) {
                obj[arr[i]] = 1;
            } else {
                obj[arr[i]]++;
            }
        }
        for (var key in obj) {
            obj[key] <= 1 && delete obj[key];
        }
        return Object.keys(obj);
    }
    console.log(getRepeat(arr));//打印结果：(3) ["1", "2", "3"]