Jest Chrome

A complete mock of the Chrome API for Chrome extensions, for use with Jest.

TypeScript support is built in. Each function and event is based on the @types/chrome package.

Getting started

npm i jest-chrome -D

Set chrome in the global scope during setup so that it is mocked in imported modules. Add a setup file to jest.config.js:

// jest.config.js

module.exports = {
  // Add this line to your Jest config
  setupFilesAfterEnv: ['./jest.setup.js'],
}

Use the setup file to assign the mocked chrome object to the global object:

// jest.setup.js

Object.assign(global, require('jest-chrome'))

Import chrome from jest-chrome for Intellisense and linting. This is the same object as chrome in the global scope.

import { chrome } from 'jest-chrome'

Usage

All of the following code blocks come from tests/demo.test.ts.

Events

Each mocked Event has all the normal methods (addListener, hasListener, hasListeners, and removeListener) plus two more: callListeners and clearListeners.

callListeners triggers a specific Event. Call callListeners with the arguments you expect Chrome to pass to your event listeners. Each event listener for that Event will be called with those arguments.

clearListeners removes all listeners for a specific Event.

test('chrome api events', () => {
  const listenerSpy = jest.fn()
  const sendResponseSpy = jest.fn()

  chrome.runtime.onMessage.addListener(listenerSpy)

  expect(listenerSpy).not.toBeCalled()
  expect(chrome.runtime.onMessage.hasListeners()).toBe(true)

  chrome.runtime.onMessage.callListeners(
    { greeting: 'hello' }, // message
    {}, // MessageSender object
    sendResponseSpy // SendResponse function
  )

  expect(listenerSpy).toBeCalledWith({ greeting: 'hello' }, {}, sendResponseSpy)
  expect(sendResponseSpy).not.toBeCalled()
})

Synchronous functions

Some Chrome API functions are synchronous. Use these like any mocked function:

test('chrome api functions', () => {
  const manifest = {
    name: 'my chrome extension',
    manifest_version: 2,
    version: '1.0.0',
  }

  chrome.runtime.getManifest.mockImplementation(() => manifest)

  expect(chrome.runtime.getManifest()).toEqual(manifest)
  expect(chrome.runtime.getManifest).toBeCalled()
})

Functions with callbacks

Most Chrome API functions do something asynchronous. They use callbacks to handle the result. The mock implementation should be set to handle the callback.

Mocked functions have no default mock implementation!

test('chrome api functions with callback', () => {
  const message = { greeting: 'hello?' }
  const response = { greeting: 'here I am' }
  const callbackSpy = jest.fn()

  chrome.runtime.sendMessage.mockImplementation((message, callback) => {
    callback(response)
  })

  chrome.runtime.sendMessage(message, callbackSpy)

  expect(chrome.runtime.sendMessage).toBeCalledWith(message, callbackSpy)
  expect(callbackSpy).toBeCalledWith(response)
})

Callbacks and chrome.runtime.lastError

When something goes wrong in a callback, Chrome sets chrome.runtime.lastError to an object with a message property. If you need to test this, set and clear lastError in the mock implementation.

Remember that lastError is always undefined outside of a callback!

lastError is an object with a getter function for the message property. If message is not checked, Chrome will log the error to the console. To emulate this, simply set lastError to an object with a getter that wraps a mock, as seen below:

test('chrome api functions with lastError', () => {
  const message = { greeting: 'hello?' }
  const response = { greeting: 'here I am' }

  // lastError setup
  const lastErrorMessage = 'this is an error'
  const lastErrorGetter = jest.fn(() => lastErrorMessage)
  const lastError = {
    get message() {
      return lastErrorGetter()
    },
  }

  // mock implementation
  chrome.runtime.sendMessage.mockImplementation((message, callback) => {
    chrome.runtime.lastError = lastError

    callback(response)

    // lastError is undefined outside of a callback
    delete chrome.runtime.lastError
  })

  // callback implementation
  const lastErrorSpy = jest.fn()
  const callbackSpy = jest.fn(() => {
    if (chrome.runtime.lastError) {
      lastErrorSpy(chrome.runtime.lastError.message)
    }
  })

  // send a message
  chrome.runtime.sendMessage(message, callbackSpy)

  expect(callbackSpy).toBeCalledWith(response)
  expect(lastErrorGetter).toBeCalled()
  expect(lastErrorSpy).toBeCalledWith(lastErrorMessage)

  // lastError has been cleared
  expect(chrome.runtime.lastError).toBeUndefined()
})

Contributions

The chrome object is based on schemas from the Chromium project, with thanks to sinon-chrome for compiling the schemas.

Special thanks to @shellscape for transferring the NPM package name jest-chrome to us!