Chromium DevTools Docs

Workflows

Checkouts

In order to make changes to DevTools frontend, build, run, test, and submit changes, several workflows exist. Having depot_tools set up is a common prerequisite.

Standalone checkout

As a standalone project, Chrome DevTools frontend can be checked out and built independently from Chromium. The main advantage is not having to check out and build Chromium.

However, to run layout tests, you need to use the chromium checkout or integrated checkout.

Checking out source

To check out the source for DevTools frontend only, follow these steps:

mkdir devtools
cd devtools
fetch devtools-frontend

Build

To build, follow these steps:

cd devtools-frontend
gclient sync
gn gen out/Default
autoninja -C out/Default

The resulting build artifacts can be found in out/Default/gen/front_end.

If you want to have faster build by disabling typecheck, consider to use devtools_skip_typecheck=true build args like:

gn gen out/fast-build --args='devtools_skip_typecheck=true'

Update to latest

To update to latest tip of tree version:

git fetch origin; git checkout origin/main  # or, alternatively: git rebase-update
gclient sync

Out of sync dependencies and cross-repo changes

The revisions of git dependencies must always be in sync between the entry in DEPS and the git submodule. PRESUBMIT will reject CLs that try to submit changes to one but not the other. It can happen that dependencies go out of sync for three main reasons:

  1. The developer attempted a manual roll by only updating the DEPS file (which was the process before migrating to git submodules, see below),
  2. after switching branches or checking out new commit the developer didn't run gclient sync, or
  3. they are working across repositories including changes in both.

In the first case, follow the manual roll process. In the second case, running gclient sync is necessary. If the changes to the submodule versions were already added to any commits (this happens when commits were created using git add -A, for example), it's necessary to unstage them (for example using git checkout -p origin/main). The latter also applies in the third case: Create a CL excluding the dependency changes and a separate CL with a proper roll.

Run in a pre-built Chromium

You can run a build of DevTools frontend in a pre-built Chromium in order to avoid the expensive Chromium build. For example, you can use the latest version of Chrome Canary, or the downloaded binary in third_party/chrome.

Running from file system

This works with Chromium 79 or later. (Requires brew install coreutils on Mac.)

To run on Mac:

<path-to-devtools-frontend>./third_party/chrome/chrome-mac/Google\ Chrome\ for\ Testing.app/Contents/Mac OS/Google\ Chrome\ for\ Testing --custom-devtools-frontend=file://$(realpath out/Default/gen/front_end) --use-mock-keychain

To run on Linux:

<path-to-devtools-frontend>./third_party/chrome/chrome-linux/chrome --custom-devtools-frontend=file://$(realpath out/Default/gen/front_end)

To run on Windows:

<path-to-devtools-frontend>./third_party/chrome/chrome-win/chrome.exe --custom-devtools-frontend=file://$(realpath out/Default/gen/front_end)

Note that $(realpath out/Default/gen/front_end) expands to the absolute path to build artifacts for DevTools frontend.

Open DevTools via F12 or Ctrl+Shift+J on Windows/Linux or Cmd+Option+I on Mac.

If you get errors along the line of Uncaught TypeError: Cannot read property 'setInspectedTabId' you probably specified an incorrect path - the path has to be absolute. On Mac and Linux, the file url will start with three slashes: file:///Users/....

Tip: You can inspect DevTools with DevTools by undocking DevTools and then opening a second instance of DevTools (see keyboard shortcut above).

Running from remote URL

This works with Chromium 85 or later.

Serve the content of out/Default/gen/front_end on a web server, e.g. via python -m http.server.

Then point to that web server when starting Chromium, for example:

<path-to-devtools-frontend>/third_party/chromium/chrome-<platform>/chrome --custom-devtools-frontend=http://localhost:8000/

Open DevTools via F12 or Ctrl+Shift+J on Windows/Linux or Cmd+Option+I on Mac.

Running in hosted mode

Serve the content of out/Default/gen/front_end on a web server, e.g. via python3 -m http.server 8000.

Then start Chromium, allowing for accesses from the web server:

<path-to-devtools-frontend>/third_party/chrome/chrome-<platform>/chrome --remote-debugging-port=9222 --remote-allow-origins=http://localhost:8000 about:blank

Get the list of pages together with their DevTools frontend URLs:

$ curl http://localhost:9222/json -s | grep '\(url\|devtoolsFrontend\)'
   "devtoolsFrontendUrl": "/devtools/inspector.html?ws=localhost:9222/devtools/page/BADADD4E55BADADD4E55BADADD4E5511",
   "url": "about:blank",

In a regular Chrome tab, go to the URL http://localhost:8000/inspector.html?ws=<web-socket-url>, where <web-socket-url> should be replaced by your desired DevTools web socket URL (from devtoolsFrontendUrl). For example, for "devtoolsFrontendUrl": "/devtools/inspector.html?ws=localhost:9222/devtools/page/BADADD4E55BADADD4E55BADADD4E5511", you could run the hosted DevTools with the following command:

$ google-chrome http://localhost:8000/inspector.html?ws=localhost:9222/devtools/page/BADADD4E55BADADD4E55BADADD4E5511

Integrated checkout

This solution is experimental, please report any trouble that you run into!

The integrated workflow offers the best of both worlds, and allows for working on both Chromium and DevTools frontend side-by-side. This is strongly recommended for folks working primarily on DevTools.

This workflow will ensure that your local setup is equivalent to how Chromium infrastructure tests your change. It comes in two flavors.

A full Chromium checkout is a pre-requisite for the following steps.

Untrack the existing devtools-frontend submodule

First, you need to untrack the existing devtools-frontend submodule in the chromium checkout. This ensures that devtools isn't dragged along whenever you update your chromium dependencies.

In chromium/src, run gclient sync to make sure you have installed all required submodules.

gclient sync

Then, disable gclient sync for DevTools frontend inside of Chromium by editing .gclient config. From chromium/src/, run

vim "$(gclient root)/.gclient"

In the custom_deps section, insert this line:

"src/third_party/devtools-frontend/src": None,

Following this step, there are two approaches to manage your standalone checkout

Flavor 1: separate gclient projects

The first approach is to have separate gclient projects, one for each repository. First, get a fresh checkout of DevTools frontend, and delete the devtools-frontend directory in that checkout.

To then create the symlink:

ln -s third_party/devtools-frontend/src path/to/standalone/devtools-frontend

Running gclient sync in chromium/src/ will update dependencies for the Chromium checkout. Running gclient sync in path/to/standalone/devtools-frontend will update dependencies for the standalone checkout.

Flavor 2: a single gclient project

The second approach is to have a single gclient project that automatically gclient sync's all dependencies for both repositories.

After removing your DevTools dependency, modify the .gclient file for chromium/src to add the DevTools project and a hook to automatically symlink (comments are optional):

solutions = [
  {
    # Chromium src project
    "name": "src",
    "url": "https://chromium.googlesource.com/chromium/src.git",
    "custom_deps": {
      "src/third_party/devtools-frontend/src": None,
    },
  },
  {
    # devtools-frontend project
    "name": "devtools-frontend",
    "url": "https://chromium.googlesource.com/devtools/devtools-frontend.git",
  }
]

Now create the symlink. In the same directory as the .gclient file, run:

ln -s src/third_party/devtools-frontend/src devtools-frontend

Do not run gclient sync before creating the symlink. If you did, remove the devtools-frontend directory as you would for Flavor 1.

Run gclient sync after creating the link to fetch the dependencies for the standalone checkout.

Chromium checkout

DevTools frontend can also be developed as part of the full Chromium checkout. This workflow can be used to make small patches to DevTools as a Chromium engineer. However, it is different to our infrastructure setup and how to execute general maintenance work, and therefore discouraged.

Checking out source

Follow instructions to check out Chromium. DevTools frontend can be found under third_party/devtools-frontend/src/.

Build

Refer to instructions to build Chromium. To only build DevTools frontend, use devtools_frontend_resources as build target. The resulting build artifacts for DevTools frontend can be found in out/Default/gen/third_party/devtools-frontend/src/front_end.

Run

Run Chrome with bundled DevTools frontend:

out/Default/chrome

Test

DevTools frontend

Test are available by running scripts in scripts/test/. Please refer to the overview document. The current test status can be seen at the test waterfall.

Layout tests

After building content shell as part of Chromium, we can also run layout tests that are relevant for DevTools frontend:

autoninja -C out/Default content_shell
third_party/blink/tools/run_web_tests.py -t Default http/tests/devtools

To debug a failing layout test we can run

npm run debug-test -- http/tests/devtools/<path>/<to>/<test>.js

The script supports either default DevTools checkout inside the chromium tree or side-by-side checkouts of chromium and DevTools. Passing --custom-devtools-frontend is not supported currently, meaning in the side-by-side scenario the DevTools checkout inside the chromium tree will be used (if not symlinked).

Creating a change

Usual steps for creating a change work out of the box, when executed in the DevTools frontend repository.

Tips to create meaningful CL descriptions:

  • Provide information on what was changed and why
  • Provide before/after screenshots (if applicable)
  • Provide relevant link to demo or example (if applicable)
  • Provide link to design doc (if applicable)

Example CL, adapted from Chromium guidelines:

Summary of change (one line)

Longer description of change addressing as appropriate:
what change was made, why the change is made, context if
it is part of many changes, description of previous behavior
and newly introduced differences, etc.

Long lines should be wrapped to 72 columns for easier log message
viewing in terminals.

How to test:
  1. ..
  2. ..

Before:  https://page-to-before-screenshot.com/before
After:  https://page-to-after-screenshot.com/after
Bug: 123456

Managing dependencies

If you need to manually roll a git dependency, it's not sufficient to update the revision in the DEPS file. Instead, use the gclient tool:

gclient setdep -r DEP@REV # for example build@afe0125ef9e10b400d9ec145aa18fca932369346

This will simultaneously update both the DEPS entry as well as the gitlink entry for the corresponding git submodule.

To sync dependencies from Chromium to DevTools frontend, use scripts/deps/roll_deps.py && npm run generate-protocol-resources. Note that this may:

  • Introduce unneeded whitespace/formatting changes. Presubmit scripts (e.g. invoked via git cl upload) will automatically fix these locally, so just apply the changes directly to your change (e.g. with git commit --amend) afterwards.
  • Introduce breaking changes to the devtools protocol, causing compilation failures. Unfortunately these need to be handled manually as there are some changes (e.g. removing an enum value) that cannot fail gracefully.

The following scripts run as AutoRollers, but can be manually invoked if desired:

  • To roll the HEAD commit of DevTools frontend into Chromium, use scripts/deps/roll_to_chromium.py.
  • To update DevTools frontend's DEPS, use roll-dep.

Merges and cherry-picks

Merge request/approval is handled by Chromium Release Managers. DevTools follows Chromium's merge criteria. In exceptional cases please get in touch with hablich@chromium.org.

Step-by-step guide on how to merge:

  1. Request approval to merge by adding the Merge-Request-XX label to the relevant crbug. A bot will come by and either ask for more info (example) or approve the request.
  2. Backmerges are done to the chromium/xxxx (e.g. chromium/3979) branch on the DevTools frontend repo. Use https://chromiumdash.appspot.com/branches or Omahaproxy to find out what branch a major Chromium version has (column true_branch).
  3. Open the to-be-merged commit in Gerrit (example).
  4. Click the hamburger menu on the top right and select “Cherry pick”.
  5. Select the branch to merge to e.g. chromium/3968.
  6. The cherry-pick CL is created (example).
  7. Get it reviewed if necessary.
  8. Once merge request approval is granted (see step 1), click the hamburger menu on the cherry-pick CL and select “Submit”. (Setting the Commit-Queue bit (+2) has no effect because these branches don’t have a commit queue.)
  9. Done.

Merge conflicts

If the approach above causes conflicts that need resolving, you can use an alternative git workflow which allows you to resolve conflicts locally before uploading. This is very similar to the chromium git merge steps but with different branch names. These steps will create the cherry-pick CL via git.

It is suggested to use the Gerrit UI approach when possible, it is more straightforward and automated. Only use this approach if your cherry-pick causes conflicts.

For the commands below, replace xxxx with the Chromium branch number that you are merging into.

To set up your local environment run:

gclient sync --with_branch_heads
git fetch
git checkout -b BRANCH_NAME origin/chromium/xxxx
git cl upstream origin/chromium/xxxx

You can then cherry-pick your commit from the main branch:

git cherry-pick -x YOUR_COMMIT

You can then resolve any conflicts, run tests, build DevTools, etc, locally to verify everything is working. Then run git cl upload to upload the CL and get a review as normal.

Make sure you remove the Change-ID: line from the description to avoid issues when uploading the CL.

Useful Commands

git cl format --js

Formats all code using clang-format.

npm run check

Runs all static analysis checks on DevTools code.