Playwright - Mock API

Hi there,
Today I want to talk about how to mock API with Playwright.

To do that, I add a new feature to the usual example. When one of the players wins, the system sends a POST request to the URL http://localhost:3001/api/winners to save the history of the results of the games.

The API server is built using json-server. You can find all the steps to reproduce the feature in this commit.

Mock API

Mocking API is a common feature in testing frameworks. In Playwright, to do that, you have to use the page.route method. This method permits you to work with API in different ways:

• Mocking responses

• Modify requests

• Modify responses

• Abort requests

Any request that a page does, including XHRs and fetch requests, can be tracked, modified and mocked in Playwright using page.route.

Mock API in action

In this article, I want to show you how to start to use this API.
Let’s start with the test. The test is straightforward; it reproduces a game where the Player X wins and, in the end, checks if the application fetches the POST request to save the winner.

The test’s base is composed in this way

test("should register the winner calling the winner API[POST]", async ({
  page,
  playerXWinMoves,
}) => {
  /// mock the API
  for (const move of playerXWinMoves) {
    await page.locator(`button:nth-child(${move})`).click();
  }
});

As you can see, the test is similar to the previous one.

Now it’s time to see how you can mock the API.

First, you have to get your API; to do this, you have to use the page.route that way.

page.route("http://localhost:3001/api/winners", async route => {   
});

So, as you can notice, the page.route method accepts two parameters: the first is the URL of your API, and the second is a function used to work with your request.

Now it’s time to get info from your request, for instance, the method and the body of the request.

page.route("http://localhost:3001/api/winners", async route => {
  const request = route.request();
  const method = await request.method();
  const postData = await request.postDataJSON();
});

Very simple, as you can see, it’s important to remember that all the methods of the request return a Promise, and you have to use the await keyword to get the value.

Now, another important feature when you want to mock API is to return custom responses.

page.route("http://localhost:3001/api/winners", async route => {
  const request = route.request();
  const method = await request.method();
  const postData = await request.postDataJSON();

  await route.fulfill({
    status: 200,
    contentType: "application/json",
    body: JSON.stringify(Object.assign({ id: 1 }, postData)),
  });
});

Using route.fulfill you can mock the response. You can set the status code, content type, body, etc. In this way, you have all the power to work with the response.

Perfect, you have all the tools to mock the request. But before closing, I want to leave you another tip. You can use the page.route as the assertion of your test.

Let’s see an example

page.route("http://localhost:3001/api/winners", async route => {
  const request = route.request();
  const method = await request.method();
  const postData = await request.postDataJSON();

  expect(await request.method()).toBe("POST");
  expect(await request.postDataJSON()).toEqual(
    expect.objectContaining({
      winner: expect.stringMatching("X"),
      createdAt: expect.any(String),
    })
  );

  await route.fulfill({
    status: 200,
    contentType: "application/json",
    body: JSON.stringify(Object.assign({ id: 1 }, postData)),
  });
});

As you can notice, you can use the expect function also inside of the page.route. So you can check if the request is called and in the right way.

To close the article, here you can find the entire test.

test("should register the winner calling the winner API[POST]", async ({
  page,
  playerXWinMoves,
}) => {
  page.route("http://localhost:3001/api/winners", async route => {
    const request = route.request();
    const method = await request.method();
    const postData = await request.postDataJSON();

    await route.fulfill({
      status: 200,
      contentType: "application/json",
      body: JSON.stringify(Object.assign({ id: 1 }, data)),
    });

    expect(await request.method()).toBe("POST");
    expect(await request.postDataJSON()).toEqual(
      expect.objectContaining({
        winner: expect.stringMatching("X"),
        createdAt: expect.any(String),
      })
    );
  });

  for (const move of playerXWinMoves) {
    await page.locator(`button:nth-child(${move})`).click();
  }
});

Ok, I think that’s all from Mocking API. Now you have all the notions to start mocking API with Playwright, and if you have any doubt, you can reach out to me without problems or check the official documentation.
I hope you enjoyed this content.

See you soon folk!

Bye bye 👋

You can find the code of this article in the GitHub repository

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