A describe() block is a function that takes two arguments:

• the name (string)
• a function that contains other describe() blocks or the actual tests.

The same applies for both Vitest and Jest.

Example of using describe block to test a class

Here is a made-up example where we have a class (referenced as service), and we are splitting up the tests for each public method:

• getBlogPosts()
• createBlogPost()

Within these describe blocks we can have multiple tests (and each test() function itself can have multiple assertions)

// Test assumes `service` is some class that needs testing
// for simplicity, its implementation is not shown here.
describe('getBlogPosts()', () => {
  test('it gets blog posts', () => {
    expect(
      service.getBlogPosts()
    ).toHaveLength(1);
  });

  test('gets archived posts', () => {
    expect(
      service.getBlogPosts({
        archived: true,
      })
    ).toHaveLength(2);
  });
});

describe('createBlogPost()', () => {
  test('it creates a blog post', async () => {
    expect(
      await service.createBlogPost({
        title: 'Hello',
      })
    ).toBe(true);
  });
});

Nested describe blocks

You can nest your describe() blocks. It is a very common pattern to have a single outer describe block (although really it often doesn't add much value).

describe('BlogService', () => {
  describe('getBlogPosts()', () => {
    describe('when posts exist', () => {
      test('it gets blog posts', () => {
        expect(
          service.getBlogPosts()
        ).toHaveLength(1);
      });
    });

    describe('when no posts exist', () => {
      test('returns empty array', () => {
        expect(
          service.getBlogPosts()
        ).toHaveLength(0);
      });
    });
  });
});

Setup and teardown

We will have a future lesson on setup and teardown functions, but just in case you wanted to see the syntax now, you can use beforeAll, beforeEach, afterEach and afterAll within describe blocks.

Each hook runs either once for all tests in that describe block, or once for each test (again within that describe block), depending on which one you use.

describe('createBlogPost()', () => {
  let service;

  beforeAll(() => {
    // in this simple  example you could do
    // this inline outside of the beforeAll too
    service = new BlogService();
  });

  beforeEach(() => {
    // Setup before each test in this describe block
    service.clearDatabase();
  });

  afterEach(() => {
    // Cleanup after each test
    service.resetState();
  });

  // also not shown: afterAll()

  test('it creates a blog post', async () => {
    expect(
      await service.createBlogPost({
        title: 'Hello',
      })
    ).toBe(true);
  });
});

If this is not clear- wait until we get to a future lesson just on this topic!