Instructions on how to verify your changes

Running scripts

Run the scripts from the root package.json (test, lint, build, start, and so on) with ./dt <script> [args] from the repository root (on Windows: .\dt <script> [args]). Don't use npm run, npm test, or npx for them. Run ./dt without arguments to list the available scripts. Arguments after the script name are passed to the script as is, so don't add a -- separator.

Testing

• To test a file, you can run ./dt test <FILEPATH> where FILEPATH is a path to a *.test.ts file, relative to the working directory.
• Test files are usually defined alongside their implementation. So if you are working on front_end/panels/timeline/TimelinePanel.ts, you would expect the test file to be defined in front_end/panels/timeline/TimelinePanel.test.ts.
• You can also test an entire directory. For example, ./dt test front_end/models/trace will run all tests in that directory.

Building & compiling

• Check for build issues by running autoninja -C out/Default.

Linting

• ./dt lint will execute ESLint and StyleLint. It will report any violations and automatically fix them where possible.
• To run the linter on a specific file or directory, you can run ./dt lint <PATH> where PATH is a path to a file or directory. This will also automatically fix violations where possible.

Fetching Tryjob Results, Failure Logs, & Screenshot Goldens

Use scripts/tools/get_cl_test_results.py to fetch all tryjob build statuses, per-test failure logs, Blink web test text diffs, and screenshot artifacts (actual_image, expected_image, image_diff) for a CL in one command:

# Fetch all build statuses, failure logs, and artifacts for a CL into an output directory:
scripts/tools/get_cl_test_results.py --cl <CL_NUMBER> --output-dir <OUTPUT_DIR>

# Filter for specific failing tests (e.g. DOMTreeWidget) or specific builders:
scripts/tools/get_cl_test_results.py --cl <CL_NUMBER> --test-filter "DOMTreeWidget" --output-dir <OUTPUT_DIR>

# Also apply Linux actual_image screenshot artifacts directly to test/goldens/linux/...:
scripts/tools/get_cl_test_results.py --cl <CL_NUMBER> --output-dir <OUTPUT_DIR> --apply-goldens

Screenshot Golden & Inspection Rules

• Linux-only goldens: Never generate, update, or commit golden PNGs from macOS. Screenshot tests in DevTools only use Linux goldens (test/goldens/linux/...) produced by Linux trybots (dtf_linux_rel / dtf_linux_dbg). Do not run screenshot tests locally on macOS to verify goldens.
• Multimodal inspection only: Always inspect actual_image.png, expected_image.png, and image_diff.png directly using your multimodal file viewer (view_file). Never write or run programmatic image comparison scripts.

Best practices

• Run tests often to verify your changes.
• Prefer using a fast build, if it exists, to keep the feedback loop shorter.
• Periodically build to check for errors.
• Run git cl presubmit -u at the end of your code changes.