@adrianhelvik/mock

@adrianhelvik/mock

I have now been using and loving this module for some time, so I decided to write some docs and share it with the world!

When to use this module

• When you do not need IE11 support for your tests
• When you want a universal mocker and stubber

What you can do with this module

• Access any deeply nested property and receive a new mock object
• Call it as a function
• Set a property to be some value
• Determine what arguments a mock or a nested is called with
• Await it

Example

import mock from '@adrianhelvik/mock'

const m = mock()

m.x.y('Hello world').z('How are you')

expect(m.x.y.$args[0]).toEqual(['Hello world'])
expect(m.x.y.z.$args[0]).toEqual(['How are you'])

m.message = 'cool brah'
expect(m.message).toEqual('cool brah')

m.fn = (who) => 'Hello ' + who

expect(m.fn('you!')).toEqual('Hello you!')
expect(m.fn('someone!')).toEqual('Hello someone!')
expect(m.fn.$args[0]).toEqual(['you!'])
expect(m.fn.$args[1]).toEqual(['someone!'])

API

mock()

This creates a mock object. All properties are unique and preserved mock objects as well. Calling the mock as a function returns a preserved mock object as well.

const m = mock()

expect(m.foo).toBe(m.foo)
expect(m()).toBe(m())
expect(m()).not.toBe(m)

$args

This property resolves to an array containing the lists of arguments for calls to this mock.

Given the following calls:

const m = mock()
m.foo(1, 2, 3)
m.foo(4, 5, 6)

.. we would get the following array when accessing m.foo.$args:

[
  [1, 2, 3],
  [4, 5, 6],
]

$isMock

This property returns true for any mock object.

const m = mock()

m.$isMock === true
m.foo.isMock === true
m.bar().$isMock === true

Promise resolution

If a mocked value is used as a promise, that's accounted for the then property returns an asynchronously resolved promise.

The promise resolves to undefined.

The good part about this is that you can use async/await and not worry about a thing!

expect(typeof mock().then).toBe('function')
const resolvedTo = await mock()
expect(resolvedTo).toBe(undefined)

Assignment

You can assign properties to a mock object. This is often very useful in testing.

const m = mock()

m.meaning.of.life = 42
expect(m.meaning.of.life).toBe(42)

Assigning functions to a mock gives you access to $args

When assigning functions as a property of a mock, you will also have access to $args of this function.

Note that the function will lose equality with the original function as it is proxied.

const m = mock()

const mockEncrypt = password => 'secret:' + password
m.encrypt = mockEncrypt
const encrypted = m.encrypt('my password')

// it uses the mock function
expect(encrypted).toEqual('secret:my password')
// and you have access to $args
expect(m.encrypt.$args[0]).toEqual(['my password'])
// , but it does not point to the same object anymore
expect(m.encrypt).not.toBe(mockEncrypt)

Changelog

v3.0.0

Made function calling optional when retrieving $args from a nested function

const m = mock()
m.foo(1).bar(2)
expect(m.foo.bar.$args).toBe(m.foo().bar.$args)