ES6 Curry

curry.js

export const curry = (fn) => {
  const r = (args) => {
    if (args.length >= fn.length) {
      return fn(...args);
    }
    return (...secArgs) => r([...args, ...secArgs]);
  };

  return (...args) => r(args);
};

curry.spec.js

describe('curry', () => {
  it('returns a curried sum function', () => {
    const sum = curry((a, b, c, d) => a + b + c + d);
    expect(sum(1, 2, 3, 4)).toEqual(10);
  });

  it('returns a function if you miss the params', () => {
    const sum = curry((a, b, c, d) => a + b + c + d);

    const sum1 = sum(1);
    const sum2 = sum1(2);
    const sum3 = sum2(3);
    expect(sum3(4)).toEqual(10);
  });

  it('combines the two approaches and still returns correct values', () => {
    const sum = curry((a, b, c, d) => a + b + c + d);

    const sum1 = sum(1, 2);
    const sum2 = sum1(3);
    expect(sum2(4)).toEqual(10);
  });
});