Higher order auto curry function

curry.js

const curryN = (fn, arity, accIn = []) => (...args) => {
  const len = args.length
  const accOut = accIn.concat(args)
  if (len + accIn.length >= arity) {
    return fn.apply(this, accOut)
  }
  return curryN(fn, arity, accOut)
}

const curry = fn => curryN(fn, fn.length)

module.exports = { curryN, curry }

curry.test.js

const { curryN, curry } = require('./curry')

describe('curryN()', () => {
  const { curryN } = factory()

  it('should correctly curry to given arity', () => {
    const threeArgs = (a, b, c = 42) => c
    const fn = curryN(threeArgs, 2)

    const res1 = fn() //=> fn with arity of 2
    const res2 = fn(1) //=> fn with arity of 1
    const res3 = fn(1, 2) //=> run function
    const res4 = fn(1, 2, 3) //=> run function
    const res5 = fn(1, 2, 3, 4) //=> run function

    expect(typeof res1).toStrictEqual('function')
    expect(typeof res2).toStrictEqual('function')
    expect(res3).toStrictEqual(42)
    expect(res4).toStrictEqual(3)
    expect(res5).toStrictEqual(3)
  })
})

describe('curry()', () => {
  it('accepts an arity of 0', () => {
    const give15 = () => 10 + 5
    const fn = curry(give15)
    expect(fn()).toStrictEqual(15)
  })

  it('accepts an arity of 1', () => {
    const add5 = n => n + 5
    const fn = curry(add5)
    expect(fn(10)).toStrictEqual(15)
  })

  it('accepts an arity of 2', () => {
    const sum = (a, b) => a + b

    const fn = curry(sum)
    const result = [fn(10)(5), fn(10, 5)]

    expect(result).toStrictEqual([15, 15])
  })

  it('accepts an arity of 3', () => {
    const sum3 = (a, b, c) => a + b + c

    const fn = curry(sum3)
    const result = [
      fn(10, 20, 30),
      fn(10)(20, 30),
      fn(10, 20)(30),
      fn(10)(20)(30)
    ]

    expect(result).toStrictEqual([60, 60, 60, 60])
  })

  it('accepts an arity of 12', () => {
    const sum12 = (a, b, c, d, e, f, g, h, i, j, k, l) =>
      a + b + c + d + e + f + g + h + i + j + k + l

    const fn = curry(sum12)
    const result = [
      fn(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12),
      fn(1, 2)(3, 4, 5, 6, 7, 8, 9, 10, 11, 12),
      fn(1, 2, 3, 4, 5, 6)(7)(8)(9, 10, 11, 12),
      fn(1, 2)(3, 4)(5, 6, 7, 8)(9, 10, 11, 12),
      fn(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)
    ]

    expect(result).toStrictEqual([78, 78, 78, 78, 78])
  })
})