const curry = fn => {
  const arity = fn.length;

  return (...args) => {
    if (args.length >= arity) {
      return fn(...args);
    } else {
      return (...secondArgs) => {
        return fn(...[...args, ...secondArgs]);
      }
    }
  }
};

const sum = (a, b) => a + b;
const addOne = curry(sum)(1);
const addNine = curry(sum)(9);

addOne(4); // 5
addNine(1); // 10

// ======== //

function partialApp(fn, ...args) {
  if (args.length === fn.length) fn(...args);
  return partialApp.bind(this, fn, ...args);
}

partialApplication(sum, 1, 2); // 3
partialApplication(sum)(1)(2); // 3
partialApplication(sum)(1, 2); // 3
partialApplication(sum, 1)(2); // 3