TDD (Test Driven Development)

Let's run our dev server and see the current state of our Todo app.

npm run dev

Open the respective dev server URL in the browser.

Now, we can immediately see a couple of things that we need to fix:

1. When we type into the input field and click the add button, the task gets added, but the input field is not cleared.
2. Click on the add button with an empty input field, and you can add an empty task, which should not happen.
3. A good to-have feature would be to add a task by pressing the enter key.

Let's fix these issues one by one while following a Test Driven Development approach.

What is TDD?

You could Google it and find a lot of definitions. Here is a simple one:

TDD is a software development process in which you write tests before you write the actual code.

Why TDD?

• It helps you to think about the problem you are trying to solve before you write the code.
• It helps you to write more confident code.
• It helps you to write more testable code.
• It helps improve accessibility.
• It helps in refactoring.

Now, let's get started with TDD.

Let's fix the issues

Declare the tests first

A good name for the tests would be:

1. should clear the input field after adding a task.
2. should not add an empty task.
3. should add a task by pressing the enter key.

// Rest of the code omitted for brevity
 
describe('Page', () => {
  // Some of the tests are omitted for brevity
 
  test.todo('should clear the input field after adding a task', () => {}); 
 
  test.todo('should not add an empty task', () => {}); 
 
  test.todo('should add a task by pressing the enter key', () => {}); 
});

What's a test.todo you ask? It's a test that is yet to be implemented. Writing all the tests and implementing them individually is a good practice. (Read more about it here).

If you run the tests now, you will see an output mentioning that the test is skipped and todo.

Let's implement them one by one.

Test: should clear the input field after adding a task

Let's implement the first test. We need to do the following:

1. Type into the input field.
2. Click on the add button.
3. Assert that the input field is cleared.

// Rest of the code omitted for brevity
 
describe('Page', () => {
  // Some of the tests are omitted for brevity
 
  test('should clear the input field after adding a task', async () => {
    const user = userEvent.setup();
    render(Page);
 
    const input = screen.getByRole('textbox', { name: 'Add Task:' });
    const button = screen.getByRole('button', { name: 'Add' });
 
    await user.type(input, 'New Task');
    await user.click(button);
 
    await waitFor(() => {
      expect(input).toHaveValue('');
    });
  });
 
  // Some of the tests are omitted for brevity
});

Make sure to remove the test.todo and replace it with test before running the tests and you should see a failing test.

Let's now write the code to make the test pass.

<script lang="ts">
  // Some of the code is omitted for brevity
 
  let taskName = '';
 
  const addTask = () => {
    tasks = [
      ...tasks,
      { id: new Date().getTime(), title: taskName, isCompleted: false },
    ];
    taskName = ''; 
  };
</script>
 
<!-- Rest of the code omitted for brevity  -->

All we did was to set the taskName to an empty string after adding the task to the list of tasks. Our tests should now pass.

Test: should not add an empty task

Let's implement the second test. We need to do the following:

1. Click on the add button.
2. Assert that the task is not added to the list of tasks.

// Rest of the code omitted for brevity
 
describe('Page', () => {
  // Some of the tests are omitted for brevity
 
  test('should not add an empty task', async () => {
    const user = userEvent.setup();
    render(Page);
 
    const input = screen.getByRole('textbox', { name: 'Add Task:' });
    const button = screen.getByRole('button', { name: 'Add' });
 
    await user.type(input, '   '); // Makes sense to also test with spaces
    await user.click(button);
 
    await waitFor(() => {
      expect(screen.queryAllByRole('listitem')).toHaveLength(0);
    });
  });
 
  // Some of the tests are omitted for brevity
});

Make sure to remove the test.todo and replace it with test before running the tests, and you should see a failing test.

Let's now write the code to make the test pass.

<script lang="ts">
  // Some of the code is omitted for brevity
 
  let tasks: Task[] = []; 
 
  let taskName = '';
 
  const addTask = () => {
    const trimmedTaskName = taskName.trim(); 
 
    if (!trimmedTaskName) return; 
 
    tasks = [
      ...tasks,
      { id: new Date().getTime(), title: trimmedTaskName, isCompleted: false }, 
    ];
    taskName = '';
  };
</script>
 
<!-- Rest of the code omitted for brevity  -->

You can already see that we unintentionally fixed an edge case where the added task name was also trimmed. The power of having to think about the problem before writing the code. We also cleared the initial data for our tasks. Our tests should now pass.

Test: should add a task by pressing the enter key

Let's implement the third test. We need to do the following:

1. Type into the input field.
2. Press the enter key.
3. Assert that the task is added to the list of tasks.

// Rest of the code omitted for brevity
 
describe('Page', () => {
  // Some of the tests are omitted for brevity
 
  test('should add a task by pressing the enter key', async () => {
    const user = userEvent.setup();
    render(Page);
 
    const input = screen.getByRole('textbox', { name: 'Add Task:' });
 
    await user.type(input, 'New Task{enter}');
 
    await waitFor(() => {
      expect(screen.queryAllByRole('listitem')).toHaveLength(1);
    });
  });
 
  // Some of the tests are omitted for brevity
});

Make sure to remove the test.todo and replace it with test before running the tests and you should see a failing test.

Let's now write the code to make the test pass.

<!-- Rest of the code omitted for brevity  -->
 
<div>
  <!-- Some of the code omitted for brevity  -->
 
  <input
    id="task-input"
    bind:value={taskName}
    on:keydown={(e) => e.key === 'Enter' && addTask()}
  />
 
  <!-- Some of the code omitted for brevity  -->
</div>

Our tests should now pass. We could further refactor this inline key-down handler into a separate function. It's pretty simple, and if our tests fail, we know we broke something.

Here's what our final code should look like:

<script lang="ts">
  // Some of the code is omitted for brevity
 
  const onKeydown = (e: KeyboardEvent) => {
    if (e.key === 'Enter') addTask();
  };
</script>
 
<div>
  <!-- Some of the code is omitted for brevity  -->
 
  <input id="task-input" bind:value={taskName} on:keydown={onKeydown} />
 
  <!-- Some of the code is omitted for brevity  -->
</div>

Great, we have now fixed all the issues. Let's rerun our tests and see if everything is working as expected.

npm run test

Great, all the tests are passing. Now, we can be confident that our Todo app will work as expected, even if we refactor the code in the future. We will do a good amount of refactoring in the next section.

At this point, your code should be a good match to the branch of the repository: 6-test-driven-development