函数科里化( add 函数)

function add() {
    let args = [].slice.call(arguments);
    let sum = function() {
        let arg = [].slice.call(arguments);
        return add.apply(null, [...args, ...arg]);
    }
    sum.toString = function() {
        return args.reduce((a, b) => a + b);
    }
    return sum;
}

console.log(add(2, 3)(4)(5)()(1)); // 15