jest-mock-curry-fn

Easier testing with curry functions + jest 🧪

Who's it for?

1. You use curried functions (see more: Curry and Function Composition)
2. You would like to mock away certain curried functions (eg. XHR calls) for ease of testing
3. You need unit tests to assert that these curried functions are called correctly
4. You don't want to write so much setup code for testing curried functions

Scenario

// makeName.ts
export const makeName = (name) => {
  const prefix = stringPrefixer('first')('second')
  const result = `${prefix} ${name}`
  return result
}

// stringPrefixer.ts
export const stringPrefixer = (first) => (second) => {
  return `${first}-${second}`
}

Scenario:

• System Under Test is makeName()
• When makeName() is called (eg. makeName('whee!)),
• Then we expect that stringPrefixer has been called with:
  • 1st mock: ('first')
  • 2nd mock: ('second')
  • Returned value: hey! whee!

Usage

// 1. import makeMockCurryFn()
import { makeMockCurryFn } from 'jest-mock-curry-fn'

// 2. import function under test
import { makeName } from './makeName'

// 3. import underlying function that requires mocking
import * as stringPrefixerImport from './stringPrefixer'

test('asserts curry fn is called', () => {
  // 4. create mocked curry fn
  const mockStringPrefixerFn = makeMockCurryFn({
    nCurriedFns: 2,
    tailFnReturnValue: 'hey!',
  })

  // 5. mock stringPrefixer() usage
  jest
    .spyOn(stringPrefixerImport, 'stringPrefixer')
    .mockImplementation(mockStringPrefixerFn.headMockFn)

  // 6. trigger usage of top-level function
  const result = makeName('whee!')

  // 7. assert curry fn calls
  mockStringPrefixerFn.expectMockFnsCalledWith('first')('second')

  // 8. assert return value
  expect(result).toEqual('hey! whee!')
})

See more: sampleUsage.test.ts

API

const options = {
  // required: number of nested curried fns, must be >= 1
  nCurriedFns: 2,
  // required: return value of last curried fn
  tailFnReturnValue: 'hey!',
}

const mockCurryFn = makeMockCurryFn(options)

const {
  // to assert curried fns to be called with certain arguments
  expectMockFnsCalledWith,
  // head fn to be used as the entrypoint for the curried fn to be used
  headMockFn,
  // tail fn to be called at the end of the chained curried fns
  tailMockFn,
  // list of mock fns (chained together to work in curry scenario)
  mockFns,
  // initial options used to initialize current curryFn
  options,
} = mockCurryFn

See more: makeMockCurryFn.ts

Tips:

• For most use cases, you will only need to use headMockFn and expectMockFnsCalledWith().
• For other advanced cases, can access individual mockFns to modify underlying jest.fn()s to suit your scenario.
  • Eg. Asserting specific mock function not to be called with certain argument.
  • Eg. Modifying tailMockFn to return different return value depending on how the function is called.