jest-mock-chain-fn

Easier testing with chained functions + jest 🧪

⭐️ Also allows snapshot testing with chained functions! ⭐️

Who’s it for?

1. You use libraries that use method chaining (eg. yargs, chalk)
   • eg. someItem.setPrice(2).setQuantity(5)
   • see more: Method Chaining
2. You need unit tests to assert that these chained methods are called correctly
3. You would like to mock away the actual implementation for ease of testing
4. You don’t want to write so much setup code for testing libraries that use method chaining

Related concepts: Builder Design Pattern, Fluent Interface

Scenario

// yargsExample.ts
// as per example usage on: https://yargs.js.org/
export const yargsExample = (yargs: any) => {
  const result = yargs
    .scriptName('pirate-parser')
    .usage('$0 <cmd> [args]')
    .command(
      'hello [name]',
      'welcome ter yargs!',
      (yargs: any) => {
        yargs.positional('name', {
          type: 'string',
          default: 'Cambi',
          describe: 'the name to say hello to',
        })
      },
      function (argv: any) {
        console.log('hello', argv.name, 'welcome to yargs!')
      }
    )
    .help().argv
  return result
}

Scenario:

• System Under Test is yargsExample()
• When yargsExample() is called,
• Then we expect that yargs has been called with:
  • scriptName() with args ['pirate-parser']
  • usage() with args ['$0 <cmd> [args]']
  • … (rest of chained functions)
  • Returned value from .argv

Usage

// 1. import makeMockChainFn()
import { makeMockChainFn } from 'jest-mock-chain-fn'
// 2. import function under test
import { yargsExample } from './yargsExample'
test('asserts chained fn is called correctly', () => {
  // 3. set .argv to return mock object
  const mockPropertyReturns = { argv: { value: { some: 'object' } } }
  // 4. create mock chain fn
  const { mockChainFn: yargs, calls } = makeMockChainFn({
    mockPropertyReturns,
  })
  // 5. trigger usage of top-level function
  const result = yargsExample(yargs)
  // 7. assert chain fn calls against snapshot
  expect(calls).toMatchSnapshot()
  // 8. assert return value
  expect(result).toEqual({ some: 'object' })
})

Key point: With minimal setup, we are able to:

1. Capture all calls made to yargs
2. Assert calls against a snapshot!

No more tedious handwiring of mocks to test method chaining. 🙃

See more: yargsExample.test.ts, chalkExample.test.ts

API

const options = {
  // optional: sets overrides for specific property keys
  mockPropertyReturns: { somePropName: {} },
}
const result = makeMockChainFn(options)
const {
  // mock that accepts method/property calls against it
  mockChainFn,
  // map of mock fns (automatically created upon call against mockChainFn)
  mocks,
  // list of calls made against mockChainFn
  calls,
  // clears mocks mapping
  clearExistingMocks,
  // clears list of calls
  clearExistingCalls,
} = result

See more: makeMockChainFn.ts

mockPropertyReturns

Used to define non-standard mock function behaviors upon property call

For: Chained function (Default scenario)

• By default, mockChainFn supports method chaining. (ie. no mockPropertyReturns setup required)
• Upon calling any arbitrary property (eg. mockChainFn.myCall()), library creates a mapping for myCall in mocks,
• The corresponding value for the myCallkey is a jest.fn() that captures all calls and returns mockChainFn() (to support method chaining).

For: Chained value (non-function)

• In order to support chained value (non-function) calls (eg. mockChainFn.compute.save),
• Need to set mockPropertyReturns = { compute: {}, save: {} }

For: Property with fixed value (function)

• Supports returning of fixed value (function) against a given property (eg. mockChainFn.myCall())
• Useful to provide own mock functions for customizable behvaior for a given function
• Eg. We want a function call to call our own mock function and return the result of our mock function
• Need to set mockPropertyReturns = { myCall: { value: jest.fn() } } (where jest.fn() is your own mock)
• Note: In this mode, a call against myCall will still be captured into calls

For: Property with fixed value (non-function)

• Supports returning of fixed value (non-function) against a given property (eg. mockChainFn.someProperty)
• Eg. We want a property call to return a mock value
• Need to set mockPropertyReturns = { someProperty: { value: 'own value' } }

Tips

• Calling jest.resetAllMocks() will clear jest.fn() mock functions that are used internally within mockChainFn (ie. in mocks mapping)
• If you want your own mock function to continue chaining, just set the return value to mockChainFn
• When defining own mock function to pass to given property, if own mock function throws error, it will be propagated upwards (ie. library does not swallow errors)