A code kata on testing webapps


  • Open in GitHub Codespaces
  • You will need to wait a bit until npm install finishes
  • Run npm start in the integrated terminal. This starts both the ‘backend’ server (port 8001) and the frontend (port 3000). In the ports tab you can open the app in the browser. Ports tab in VSCode

Alternatively you can also set up the project locally:

  • Make sure NodeJS 18.12.1 has been installed. You can also use nvm and execute nvm install and nvm use.
  • Execute npm install
  • You can then execute npm start to run backend server (port 8001) and frontend (port 3000) locally.


Screenshot of Todo Application

The application allows you to create todos, mark them as done, delete them and see how many todos are left.


1. Get familiar with Jest

To write our unit tests, we will be using Jest. This is a unit testing framework for JavaScript/TypeScript.

Close-up of Add Todo

Notice how the “Add” button is disabled when the description is emtpy. It remains empty if the user enters whitespace. This logic is done in the isValid function in src/components/add-todo/index.tsx.

Create tests in src/components/add-todo/add-todo.test.tsx:

Note: Tests are typically co-located with the actual implementation and end in .test.tsx or .spec.tsx.

  • when passing an empty string, isValid should return false
  • when passing a string with just whitespace, isValid should return false
  • when passing a string with text, isValid should return true

Keep the expect documentation and api documentation handy.

To execute the tests run npm test. This will start Jest in “watch” mode. Whenever you save a file, the tests will run. Press w to explore the watch mode

  • try only watching the add-todo.test.tsx file
  • try focusing on a single test
  • switch back to running all tests
  • run only the tests related to files changed since last commit

If you want to debug a test in VSCode, you can set a breakpoint and press F5. The first time you might need to select the “Debug Test” launch configuration. Adding console.log statements is also a great debugging tool ?

2. Test builders

Close-up of number of todos left

At the bottom of the todo app you will see the number of todos that are left. As you toggle todos, this number will change. The number is calculated in the countTodosLeft function (in src/components/footer/index.tsx). It takes an array of todos and returns how many todos are still left.

Create tests in footer.test.tsx:

  • when passing an empty array, countTodosLeft should return 0
  • when passing an array where all todos are done, countTodosLeft should return 0
  • when passing an array where 1 todo is done and 2 are not done, countTodosLeft should return 2

A todo looks as follows:

const todo = {
    id: "3044efbc-7e54-4751-a96c-e01474caf8a7",
    description: "Jog around the park",
    done: false,

To generate ids and descriptions you can use faker. Check out the api documentation to see what it can do

import { faker } from "@faker-js/faker";
const id = faker.datatype.uuid();

You might also want to create a test builder to easily create todos. A pattern you can use in JavaScript/TypeScript is based on the spread syntax.

function anAddress(overrides: Partial<Address> = {}): Address {
    return {
        street: faker.address.street(),
        zipCode: faker.address.zipCode("####"),

const randomAddress = anAddress();
const addressInTheCity = anAddress({ zipCode: "2000" });

Note: You can keep the test-builder in the test file, or put it in a separate test-builders file in src/test-lib.

3. Testing Library

Close-up of "All done" footer

You might notice that the application shows

  • “All done!” when there are no todos left
  • “1 todo left” when there is one
  • “4 todos left” when there are multiple

Write some tests in footer.test.tsx to cover these cases. This time the functionality is in a React component. We can use testing-library to render a React component in our tests.

Note: Testing-library also works on regular DOM applications, and with other frameworks (Vue, Angular,..)

Import screen and render from our test-lib. In a test you can render the Footer component with an array of todos.

import { screen, render } from "../../test-lib/test-utils";
test("show todos", () => {
    const exampleTodos = [aTodo({ done: true })];
    render(<Footer todos={exampleTodos} />);

Note: Wondering why we import from test-lib/test-utils instead of @testing-library/react? This setup is recommended to reduce certain boilerplate code.

Use the testing-library queries to find the DOM element. Unsure which query would work best? Try putting screen.logTestingPlaygroundURL() after the render method and go to the url.

  • Make sure you understand the difference between getBy.., queryBy.. and findBy...

These queries return DOM elements which you might want to assert. (is it present, is a button disabled, …). Go over jest-dom to see what matchers can help you.

Note: Aren’t we running these tests in NodeJS? How can we render DOM elements without a browser? We’re testing against a fake implementation of the DOM called jsdom. It can’t implement/simulate everything, but works great for these tests.

4. Simulating the user

After a user adds a todo, we clear the “Description” field to allow the user to quickly add a new todo.

Create tests in add-todo.test.tsx:

  • user types something in ‘Description’, presses enter: description should become empty
  • user types something in ‘Description’, clicks the add button: description should become empty

Just like in the previous exercise, we can use testing-library to find and verify the DOM elements. This time we also want to trigger DOM events to simulate the user. However we want to avoid coupling our test to a specific choice in the implementation. In the test, we don’t care if the component is using onChange/onKeyDown/onKeyUp/onSubmit/…

We’ll use user-event instead. It will simulate all the correct DOM events when simulating a user action. The render function from test-lib/test-utils has been setup to return a userEvent object.

test("user hovers over a button", async () => {
    const { user } = render(<AddTodoForm />);
    const coolButton = screen.getByRole("button", { name: /my cool button/i });
    await user.hover(coolButton);

Note: the methods on user return promises, you will need to await them.

You might notice that the “Description” field is only cleared after submitting to the server. You can use waitFor for this. (eg: await waitFor(() => expect(...).toHaveValue(...)))

5. Faking the backend

When the user adds a todo in the <AddTodoForm> component (doing a POST to /api/todos), the <TodoList> will show this new todo. A user can then mark the todo as done or delete it. We would like to test if this actually happens:

Create tests in src/app.test.tsx:

  • user types something in the description and clicks add. The todo shows up in the list.
  • start off with a single todo item in the list. When the user marks it as done, the footer should show ‘All done!’
  • start off with a single todo item in the list. When the user deletes it, the text ‘Add some todos’ should show up

The components we are testing are executing REST calls. Luckily we are not testing against the real backend. In src/test-lib/test-server.ts msw is being used to fake our backend. In src/setupTests.ts we make sure that we cleanup after every test.

Extra tips:

  • Before rendering, call resetTodos to start the test with a specific todo item. This way you don’t have to add it through the UI in every test.
  • We are missing a call for removing todo items. Make sure the test-server also implements DELETE /api/todos/:todoId. Currently our web-app always requires a JSON response. HTTP reponses without a body are not supported.
  • Even if the server is fake, the results won’t be “instant”. The await screen.findBy-queries will come in handy. docs
  • You can scope testing-library queries using within. (eg, within(list).getBy...)
  • The todo items have a trash icon, but it does have accessible text so it is still easy to find. A UI that is not accessible, will be hard to test.

6. Stubbing the backend

Close-up of Add todo form with error message

We would like to test situations where the server returns an error. We could expand the fake test-server implementation, but that could lead to an overly complex fake implementation. Instead we will temporarily replace an endpoint with a stub in our tests. The POST /api/todos should return a 400 status code and a json response that looks like this {"code":"description_too_long","errorMessage":"Description may not be more than 100 characters"}

Create tests in add-todo.test.tsx:

  • given the user adds a todo, when the server returns an error, then the error message shows up
  • given the user adds a todo, when the server returns an error, then description input text is not cleared

You can temporarily override a call by calling mswServer.use(). These overrides are cleaned up after each test (see src/setupTests.ts).

For example

import { rest } from "msw";
import { mswServer } from "../../test-lib/test-server";

test("my test", () => {
        rest.get('/api/todos', (req, res, ctx)=> {...})
    ); // similar to `test-server.ts`

7. Mocking the backend

Close-up of Add Todo

We would like to verify if the Add button actually calls the backend. Does it do a POST with the correct parameters?

Create tests in add-todo.test.tsx:

  • when the user adds a todo, a POST call is done to /api/todos with {"description":"..."} in the body.

Jest allows us to create and verify mock functions. We can combine this with the previous exercise to verify a call was executed.

const postCall = jest.fn();
    rest.get('/api/todos', (req, res, ctx)=> {
        // use postcall
expect(postCall).toHaveBeenCalledWith({ description: expectedDescription })

Bonus rounds

Screenshot of "Something broke" error screen

  • when the GET call to /api/todos fails, a message should show up with “Something broke”.
  • when toggling a todo item, a checked css class should be applied to the todo item.
  • when toggling a todo item, the checkbox is checked optimistically. However if the PUT call fails, the todo item should be unchecked
  • while adding a todo, the ‘Add’ button should be disabled and show ‘Adding…’. You might need to delay the server to verify this.
  • create a helper method mockHandler that takes a jest.Mock<any, any> and returns a ResponseResolver<RestRequest, RestContext> to easily mock REST calls: For example:

const postCall = jest.fn().mockReturnValue({ id: 3 });
mswServer.use("/api/contacts", mockHandler(postCall)));
// ...
expect(postCall).toHaveBeenCalledWith({ name: "Burt" });


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