Is there any way to mock private functions with Jest?

Question

The ES6 module that I want to test looks as follows:

function privateFunction() {
   ...
}
export function publicFunction() {
   ... does something ...
   privateFunction()
   ... does something else ...
}

I am using JEST for my unit tests and I am trying to find a way to test publicFunction and avoiding the execution of privateFunction by mocking it but I couldn't succeed in the mock attempt. Any idea?

Answer 1

I found out a way to mock my private function by using the babel-plugin-rewire module.

In package.json I have the following:

  "devDependencies": {
    ...
    "babel-plugin-rewire": "1.0.0-beta-5",
    "babel-jest": "18.0.0",
    ...

In .babel.rc I have the following:

{
  "presets": [
    "es2015",
    "stage-0",
    "react"
  ],
  "env": {
    "test": {
      "plugins": [
        "babel-plugin-rewire"
      ]
    }
  },
  ...

At this point I was able to mock the private function:

import * as moduleToTest from './moduleToTest.js'

describe('#publicFunction', () => {
  it('mocks private function', () => {
    moduleToTest.__Rewire__('privateFunction', () => { console.log('I am the mocked private function') })
    ...
  })
})