function func(){/* ......  */}

func(3, 5) // 8
func(3)(5) // 8

function func() {
    var a = arguments;
    return a.length > 1 ? a[0] + a[1] : function(x) { return a[0] + x; };
}

function func(a, ...args) {
  if(args.length === 0)
    return func.bind(this, a);
  return args.reduce((summ, val) => summ + val, a)
}

const add = function add(...params) {
	const value = params.length === 1 ? params.shift() : eval(params.join('+'));
	const fn = (...params) => add(value, ...params);
	fn.valueOf = Number.bind(null, value);
	return fn;
}

console.log(+add(1,3));
console.log(+add(1)(3));
console.log(+add(4,5,1)(3,7)(1,1,1,1,1,1,1,1,1,1,1,1)(0,8,10));
console.log(+add(1)(2)(3)(-6));