Guide on end-to-end testing

This directory hosts the end-to-end tests we run on DevTools. These tests open a target page and a DevTools frontend page, for which the DevTools frontend connects to the target page over CDP. We use Puppeteer to talk over CDP and all functionality of Puppeteer is available to you as well when writing end-to-end tests. We use Mocha as testing framework.

The goal of these end-to-end tests is to implement core user journeys throughout the application. As such, the tests you write should read like a little story that you can read, even if you don't know how it is implemented.

The tests therefore have a dual purpose:

1. Verify that core user stories are working as intended and are not broken by a particular DevTools frontend change.
2. Serve as documentation and reference point for how DevTools is intended to be used.

Running tests

The following command (builds and) runs all tests:

npm run auto-e2etest

Note, you can use it.only to run a single test!

Flags

The following list of flags are commonly used:

• --jobs=N — use N parallel runners to speed things up
  • The number to chose depends on several factors, not just the number of cores. A good way to determine the number is to bisect the number of cores until the tests pass.
• --chrome-binary-path=LOCATION — set a path to the chrome executable
• --chrome-features=FEATURES — set a comma separated list of chrome features passed as --enable-features=[FEATURES] to the chrome binary.
• --test-file-pattern=FILE_PATTERN — run tests in selected test files only. The extglob pattern matches paths relative to the test/e2e/ directory. To run all sources panel tests, for example, use --test-file-pattern=sources/*.

To use the flags, first append -- to the npm command. For example,

npm run auto-e2etest -- --jobs=4

See scripts/test/run_test_suite.py for more flags.

Environment variables

The following environmental variable are commonly used:

• ITERATIONS=N - Runs every test N number of times.
• LATE_PROMISES=true|N - Delays all promises in the frontend by N number of milliseconds. Defaults to 10ms if set to true.
• STRESS=true - Emulates CPU slow-down.

Skipping tests

You can disable a test for all platforms using it.skip. If you are disabling a flaky test, consider disabling it only on the affected platforms. For example,

it.skipOnPlatforms(['mac', 'win32'], '[crbug.com/xxx] ...', () => {...});
it.skipOnPlatforms(['linux'], '[crbug.com/xxx] ...', () => {...});

To use skipOnPlatforms, you need to import it from test/shared/mocha-extensions.ts.

Debugging tests

To see what the test script does, run npm run debug-e2etest. This will bring up the chrome window and stop just before your test script is about to execute. The test will then run to completion and exit. You can add an infinite await await new Promise(() => {}); at the end of your test to give you some time to examine the result of your test script.

The it.repeat helper is useful for reproducing a flaky test failure. e.g.

it.repeat(20, 'find element', async () => {...});

it.repeat behaves like it.only in that it will cause just that single test to be run.

Debug tests with DevTools

Running npm run debug-e2etest also allows debugging a test with DevTools. Please note that there are two different targets that can be inspected, and the way they are inspected differs slightly:

• You can debug the “DevTools under test” with DevTools-on-DevTools. Use the standard DevTools key combination to open another DevTools instance while you look at the “DevTools under test”. You can set breakpoints and inspect the status of the “DevTools under test” this way.
• You can debug the puppeteer side by inspecting the Node.js process that runs the e2e suite. Either open chrome://inspect or click the Node.js icon in any open DevTools window to connect to the puppeteer process. You can step through the puppeteer test code this way.

Debug tests with VSCode

To debug in VSCode, open the “Run and Debug” sidebar, select “Run end-to-end tests in VS Code debugger” from the dropdown, and click the start button or press F5. Current limitations when using VSCode:

• VSCode only attaches to the node portion of the code (mostly the test files and the test helpers), not to Chrome.
• VSCode debugging only works with headless mode.

Debugging flaky tests

Sometimes tests may fail in a seemingly random way. This tends to happen in our CI due to difference in hardware. To debug this, you can use stressor bots which are specially-made for this purpose. These bots are similar to normal bots except their environment variables are configurable.

The following command runs the stressor bot on all files of the latest commit with reasonable settings:

git cl try -B devtools-frontend/try \
  -b e2e_stressor_linux \
  -p e2e_env="{ \
    \"TEST_PATTERNS\": \"$(git diff-tree --no-commit-id --name-only HEAD -r | grep test/e2e | cut -c 10- - | tr "\n" ",")_\", \
    \"STRESS\": true, \
    \"LATE_PROMISES\": true, \
    \"ITERATIONS\": 20 \
  }"

By default, tests are run using the debug build. To run it with the release build, append -p builder_config=Release to the end of the command.

Please use a reasonable number of iterations and include the minimal amount of test files to avoid overloading the bots. This bot is experimental and the parameters might change in the future.

General implementation details

To that end, the “what” from the “how” are separate in end-to-end tests. The “what” is the actual end-to-end test. The “how” are functions in helpers that implement the interaction with DevTools to perform a particular action.

For example, an end-to-end test might read like this:

it("can show newly created snippets show up in command menu", async () => {
  await openSourcesPanel();
  await openSnippetsSubPane();
  await createNewSnippet("New snippet");

  await openCommandMenu();
  await showSnippetsAutocompletion();

  assert.deepEqual(await getAvailableSnippets(), ["New snippet\u200B"]);
});

The test describes the user journey and then describes what actions the user takes to fulfill that journey. For example, when the user wants to “open the command menu”, that is an action performed. The implementation is separated in a helper that implements “how” the user performs that action.

The separation of the two concepts allows us to change the underlying implementation of the action while making sure that the user journey remains intact.

For example, this is the implementation of openCommandMenu():

export const openCommandMenu = async () => {
  const { frontend } = getBrowserAndPages();

  switch (platform) {
    case "mac":
      await frontend.keyboard.down("Meta");
      await frontend.keyboard.down("Shift");
      break;

    case "linux":
    case "win32":
      await frontend.keyboard.down("Control");
      await frontend.keyboard.down("Shift");
      break;
  }

  await frontend.keyboard.press("P");

  switch (platform) {
    case "mac":
      await frontend.keyboard.up("Meta");
      await frontend.keyboard.up("Shift");
      break;

    case "linux":
    case "win32":
      await frontend.keyboard.up("Control");
      await frontend.keyboard.up("Shift");
      break;
  }

  await waitFor(QUICK_OPEN_SELECTOR);
};

As you can see, the way the user opens the command menu is via key-bindings. We don‘t “bypass” the functionality by calling functions on components or on our models directly; we instruct Puppeteer to do exactly what a user would do. Doing so, we are certain that we don’t test our component abstractions and potentially lose track of integration issues.

Secondly, the function has a waitFor, which waits for the command menu (found by the QUICK_OPEN_SELECTOR) to appear. For every action that is performed in DevTools, there must be a corresponding user-visible change in the UI. This means that you always have to wait for something to happen and you can't assume that, as soon as you have performed an action, the UI has updated accordingly.

Note: Because of the async rendering of DevTools, content might not be strictly visible when DOM Nodes are appended to the DOM. As such, be aware of the functionality you are testing and relying on, as it could render differently than you originally assumed.

To summarize:

1. Separate the “what” from the “how”.
2. Use real actions (clicking, using key-bindings, typing) instead of “bypassing” via components/models.
3. Every action must be observed by a change in the UI and must be waited for.
4. Be aware of the async rendering of DevTools

Helpers

There are two kinds of helpers:

1. Helpers written as part of the end-to-end test, implementing the “how” of user actions.
2. Helpers supplied to interact with a page and abstract away the way the tests are run.

The former are implemented in helpers, written by the DevTools maintainers and are specific to the implementation of the DevTools frontend. The latter are implemented in ../shared, written by the Puppeteer maintainers and are predominantly DevTools-agnostic (apart from the handling of a target page + CDP connection).

In general, the e2e/helpers make use of the shared helpers. See ../shared/README.md for more documentation on the shared helpers.