Unit testing Chrome Extensions

Unit testing allows small sections of code to be tested in isolation from the rest of your extension, and outside of the browser. For example, you could write a unit test to ensure that a helper method correctly writes a value to storage.

Code written without using extension APIs can be tested as normal, using a framework such as Jest. To make code easier to test this way, consider using techniques such as dependency injection which can help to remove dependencies on the chrome namespace in your lower level implementation.

If you need to test code which includes extension APIs, consider using mocks.

# Example: Using mocks with Jest

Create a jest.config.js file, which declares a setup file that will run before all tests:

module.exports = {
  setupFiles: ['<rootDir>/mock-extension-apis.js']
};

In mock-extension-apis.js, add implementations for the specific functions you expect to call:

global.chrome = {
  tabs: {
    query: async () => { throw new Error("Unimplemented.") };
  }
};

Then, use jest.spy to mock a return value in a test:

test("getActiveTabId returns active tab ID", async () => {
  jest.spyOn(chrome.tabs, "query").mockResolvedValue([{
    id: 3,
    active: true,
    currentWindow: true
  }]);
  expect(await getActiveTabId()).toBe(3);
});

# Next steps

To ensure your extension functions as expected, we recommend adding end-to-end tests. See Testing Chrome Extensions with Puppeteer for a complete tutorial.