DEV Community

Vimalraj Selvam
Vimalraj Selvam

Posted on • Originally published at on

Test the Theme toggle app

In my previous article we’ve built an app with Theme toggling capability. In this article, let’s test that feature.

I’ll be using react-testing-library and jest-dom as a partner library for the react-testing-library for leveraging custom DOM element matchers for Jest.

Let’s install both the libraries as dev dependencies.

npm install -D react-testing-library jest-dom

Since we used create-react-app, there is a nice article to get started with running tests in their doc site here. I highly recommend my readers to read through the doc once.

As mentioned here, let’s create a setupTests.js under the src directory.

import "react-testing-library/cleanup-after-each";
import "jest-dom/extend-expect";

  • The 1st import ensures the rendered component by react-testing-library is removed after every test ran.
  • The 2nd import let’s us use the jest-dom‘s custom matchers mainly we use it for assertion.

Before writing our tests, let’s add an attribute data-testid in the header and button component in our App.js component as like follows:

<header className="App-header" style={{ backgroundColor: theme.backgroundColor,
color: theme.color }} data-testid="header">
  <button type="button"
    onClick={toggle} style={{ backgroundColor: theme.backgroundColor, color: theme.color, outline: 'none' }} data-testid="toggle-theme-btn" > 

We’ll be using the data-testid attribute to find the elements in our tests.

Now let’s create a new test. Go and delete everything on App.test.js and have the following test:

import React from "react";
import { render } from "react-testing-library";
import App from "./App";

test("renders with light mode default", () => {
  const { getByTestId } = render(<App />);
  expect(getByTestId("header")).toHaveStyle("background-color: white");

In our 1st line, we render our App component and then we assert whether there is a Toggle theme button and the default background color is white using the respective data-testids.

Let’s run the test and see whether the run is success.

npm run test

Hoorah!!! The test ran successfully!

Now, let’s add another test to validate whether clicking on the toggle button changes the background color.

test("toggles the theme", () => {
  const { getByTestId } = render(<App />);
  const toggleBtn = getByTestId("toggle-theme-btn");;
  expect(getByTestId("header")).toHaveStyle("background-color: black");;
  expect(getByTestId("header")).toHaveStyle("background-color: white");

Make sure you import fireEvent from the react-testing-library.

Let’s see the run result. The newly added test failed 😟. Why?

✕ toggles the theme (29ms)

  ● toggles the theme


    - Expected

    - background-color: black;
    + background-color: white;

      13 | const toggleBtn = getByTestId('toggle-theme-btn')
      14 |
    > 15 | expect(getByTestId('header')).toHaveStyle('background-color: black')
         | ^
      16 |
      17 | expect(getByTestId('header')).toHaveStyle('background-color: white')
      18 | })

      at Object.toHaveStyle (src/App.test.js:15:33)

The execution stack trace says that the background-color is still white. That means after firing a click event, the localStorage‘s setItem method is not properly triggered.

To make this work, we should mock our localStorage since the test doesn’t execute on the real browser and don’t have the Storage. Now open up your setupTests.js and append the following lines:

let store = {};

// Mock the `localStorage.getItem` method to return the value stored in the given key
jest.spyOn(Storage.prototype, "getItem").mockImplementation(key => {
  return store[key];

// Mock the `localStorage.setItem` method to insert a given value into the given key
jest.spyOn(Storage.prototype, "setItem").mockImplementation((key, value) => {
  return (store[key] = value + "");

// Mock the `localStorage.clear` method to clear the `store`
jest.spyOn(Storage.prototype, "clear").mockImplementation(() => {
  store = {};

Now let’s run our test and see the results. Still failing for the same reason. Let’s add some debug points with our famous console.log statements in our theme-context.js.

I have added the following statement in our toggle method within our ThemeProvider function:

console.log(">>> clicked");

Let’s see the run results and notice whether that statement is printed. Surprisingly, not!

The reason is our App component in the test is not wrapped with ThemeProvider. We should wrap it. Let’s do that.

Once you wrapped, your test must pass and the added console.log statement should print twice!!!

Great work. But do we need to wrap the App component with ThemeProvider in every single test. I heard you saying that’s a pain. But we have a solution for that too.

So I’ve asked this question to Kent in twitter here. And I got immediate responses.

Yep. I recommend wrapping, but moving works too. Learn more about wrapping without the pain here:

So as per the above, let’s create a test utility file to define our own render method. Create a file called src/test-util.js and add the following:

import React from "react";
import { render } from "react-testing-library";
import { ThemeProvider } from "./theme-context";

const AllMyProviders = ({ children }) => {
  return <ThemeProvider>{children}</ThemeProvider>;

const MyCustomRender = (component, options) =>
  render(component, { wrapper: AllMyProviders, ...options });

// re-export everything from the `react-testing-library`
export * from "react-testing-library";

// export our custom render method
export { MyCustomRender as render };

Let’s go to our test and make few changes:


import { render, fireEvent } from "react-testing-library";


import { render, fireEvent } from "./test-utils";

And then, remove the ThemeProvider wrapping for the App component in all the tests. Now execute your test and see.

The tests should run fine. That’s it for today. If you found this post useful, please hit like and share it.

Top comments (0)