curry multi add

curry.js

function curry(fn) {
  var res;
  function curried() {
    args = [].slice.call(arguments);

    if (!res && args.length == fn.length) {
      res = fn.apply(this, args);
      return curried;
    } else if (!res && args.length > fn.length) {
      res = fn.apply(this, args);
      return curried.apply(this, args.splice(fn.length));
    } else if (res && args.length == fn.length - 1) {
      res = fn.apply(this, [res].concat(args));
      return curried;
    } else if (res && args.length > fn.length - 1) {
      res = fn.apply(this, [res].concat(args));
      return curried.apply(this, args.splice(fn.length - 1));
    } else {
      return function() {
        return curried.apply(this, args.concat([].slice.call(arguments)));
      };
    }
  }

  curried.valueOf = function() {
    var _res = res;
    res = null;
    return _res;
  };

  return curried;
}

function add(a, b) {
  return a + b;
}

let curriedAdd = curry(add);
curriedAdd(1)(2)(3); // 6
curriedAdd(1)(2, 3)(4, 5, 6); // 21