partial application

function sum(...args) {
  return args.reduce((acc, num) => acc + num);
}

function partial(fn, ...args) {
  function invokeNext(args) {
    return (arg) => {
      if (arg !== undefined) {
        return invokeNext([...args, arg]);
      } else {
        return fn.apply(null, args);
      }
    }
  }
  return invokeNext(args);
}