SOURCE

function sumWithES6(...rest) {
    var _args = rest;

    var _adder = function (...innerRest) {
        _args.push(...innerRest); // 这里使用的是ES6数组的解构
        return _adder;
    };

    _adder.toString = function () {
        let sum = _args.reduce(function (a, b) {
            return a + b;
        });
        return sum;
    };
    return _adder;
}

console.log(sumWithES6(1)(2)(3)); // 6