js 链式

写一个函数让下面两个输出结果相同；
console.log(sum(2)(3));
console.log(sum(2,3));
 

var sum = (function() {
    var list = [];
    var add = function() {
        // 拼接数组
        var args = Array.prototype.slice.call(arguments);
        list = list.concat(args);
        return add;
    }
    // 覆盖 toString 方法
    add.toString = function() {
        // 计算总和
        var sum = list.reduce(function(pre, next) {
            return pre + next;
        });
        // 清除记录
        list.length = 0;
        return sum;
    }
    return add;
})();
var s=sum(2, 3 ,4)(3)(1,2);
console.log(s.toString());
console.log(sum(2, 3 ,4).toString());
console.log(sum(2, 3 ,4).toString());
console.log(sum(2)(3)(4).toString());
console.log(sum(2)(3)(4).toString());