TDD'd Currying

curry.js

export default function curry(func, ...args) {
  function inner(currentArgs, ...newArgs) {
    const currentLength = currentArgs.length + newArgs.length;

    if (currentLength >= func.length) {
      return func(...currentArgs, ...newArgs);
    } else {
      return (...furtherArgs) =>
        inner([...currentArgs, ...newArgs], ...furtherArgs);
    }
  }

  return inner(args);
}

curry.test.js

import curry from "./curry";

function target(a, b, c) {
  return a + b + c;
}

function target2(a, b, c, d, e) {
  return a + b + c + d + e;
}

describe("unit: curry", () => {
  let bob;

  beforeEach(() => {
    bob = curry(target);
  });

  it("allows you to supply all arguments at once", () => {
    expect(bob(1, 2, 3)).toBe(6);
    expect(bob(3, 6, 9)).toBe(18);
  });

  it("allows you to set initial args in the curry function", () => {
    bob = curry(target, 1);

    expect(bob(2, 3)).toBe(6);
    expect(bob(3, 5)).toBe(9);
  });

  it("allows you to supply all arguments seperately", () => {
    expect(bob(1)(2)(3)).toBe(6);
    expect(bob(3)(6)(9)).toBe(18);
  });

  it("allows you to supply all arguments seperately with an initial curry", () => {
    bob = curry(target, 1, 2);

    expect(bob(3)).toBe(6);
    expect(bob(9)).toBe(12);

    bob = curry(target2, 1, 2);

    expect(bob(3, 4, 5)).toBe(15);
    expect(bob(9, 10, 11)).toBe(33);
  });

  it("allows you to recurry", () => {
    bob = curry(target2, 1, 2);
    const bob2 = curry(bob, 3);

    expect(bob2(4, 5)).toBe(15);

    expect(curry(bob2, 9, 10)).toBe(25);
  });
});