How to mock third party modules with Jest

Question

I've got current import in my test target:

import sharp from 'sharp'

and using it with in my same test target:

return sharp(local_read_file)
    .raw()
    .toBuffer()
    .then(outputBuffer => {

In my test, I'm doing below to mock sharp functions:

jest.mock('sharp', () => {
  raw: jest.fn()
  toBuffer: jest.fn()
  then: jest.fn()
})

but I'm getting:

  return (0, _sharp2.default)(local_read_file).
                             ^
TypeError: (0 , _sharp2.default) is not a function

Is there a way we can mock all Sharp module functions using Jest with the function?

Answer 1

You need to mock it like this :

jest.mock('sharp', () => () => ({
        raw: () => ({
            toBuffer: () => ({...})
        })
    })

First you need to return function instead of an object, cause you call sharp(local_read_file). This function call will return an object with key raw which holds another function and so on.

To test on the every of your functions was called you need to create a spy for every of the function. As you can't to this in the initial mock call, you can mock it initially with a spy and add the mocks later on:

jest.mock('sharp', () => jest.fn())

import sharp from 'sharp' //this will import the mock

const then = jest.fn() //create mock `then` function
const toBuffer = jest.fn({()=> ({then})) //create mock for `toBuffer` function that will return the `then` function
const raw = jest.fn(()=> ({toBuffer}))//create mock for `raw` function that will return the `toBuffer` function
sharp.mockImplementation(()=> ({raw})) make `sharp` to return the `raw` function