Sum multiple notations

sum-multiple-notations.js

function sum(...args) {
    var resp = args.length==1 ? args[0] : args.reduce((key, acc) =>acc+=key,0);
    function innerSum(...args1) {
        let b = args1.length==1 ? args1[0] : args1.reduce((key, acc) =>acc+=key,0);
        resp += b;
        return innerSum;
    }
    innerSum.toString = function(){
        return resp;
    }
    return innerSum;
}

sum(1)(2)(3)(4)(5)(6);
sum(1,2,3,4,5,6);
sum(1,2)(3)(4,5,6);

Write a generic function Sum.
Which returns sum of all numbers on calling it in different fashion. How do you implement this?

Sum(1)(2)(3)(4)(5)(6) - 21
Sum(1,2)(3)(4,5,6) - 21
Sum(1,2,3,4,5,6) - 21