A lightweight Jest mock for unit testing Express

Installation:

yarn add --dev @jest-mock/express

npm install --save-dev @jest-mock/express

Importing:

import { getMockReq, getMockRes } from '@jest-mock/express'

getMockReq is intended to mock the req object as easily as possible. In its simplest form you can call it with no arguments to return a standard req object with mocked functions and default values for properties.

const req = getMockReq()

To create mock requests with specific values, you can simply provide them to the function in any order, with all being optional. The advantage of this is that it ensures the other properties are not undefined. Loose type definitions for standard properties are provided, custom properties ([key: string]: any) will be passed through to the returned req object.

// an example GET request to retrieve an entity
const req = getMockReq({ params: { id: '123' } })

// an example PUT request to update a person
const req = getMockReq({
  params: { id: 564 },
  body: { firstname: 'James', lastname: 'Smith', age: 34 },
})

getMockRes will provide a res object with Jest mock functions. Chaining has been implemented for the applicable functions.

const { res, next, clearMockRes } = getMockRes()

All of the returned mock functions can be cleared with a single call to mockClear. An alias is also provided called clearMockRes.

const { res, next, clearMockRes } = getMockRes()

beforeEach(() => {
  clearMockRes()
})

It will also provide a mock next function for convenience. That will also be cleared as part of the call to mockClear/clearMockRes.

To create mock responses with values, you can simply provide them to the function in any order, with all being optional. Loose type definitions for standard properties are provided, custom properties ([key: string]: any) will be passed through to the returned res object.

const { res, next, clearMockRes } = getMockRes({
  locals: {
    user: getLoggedInUser(),
  },
})

A full example could be:

// generate a mocked response and next function, with provided values
const { res, next } = getMockRes({
  locals: {
    isPremiumUser: true,
  },
})

test('will respond with the entity from the service', async () => {
  // generate a mock request with params
  const req = getMockReq({ params: { id: 'abc-def' } })

  // provide the mock req, res, and next to assert
  await myController.getEntity(req, res, next)

  expect(res.json).toHaveBeenCalledWith(
    expect.objectContaining({
      id: 'abc-def',
    }),
  )
  expect(next).toBeCalled()
})