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 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
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
gclient sync

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.)

<path-to-chrome>/chrome --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 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 (F12 on Windows/Linux, Cmd+Option+I on Mac).

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-chrome>/chrome --custom-devtools-frontend=http://localhost:8000/

Open DevTools via F12 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 python -m http.server.

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

<path-to-chrome>/chrome --custom-devtools-frontend=http://localhost:8000/ --remote-debugging-port=9222

In a regular Chrome tab, go to the URL http://localhost:9222#custom=true. It lists URLs that can be copied to new Chrome tabs to inspect individual debug targets.

Integrated checkout

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.

Remove existing devtools-frontend sub-repository

First, you need to remove the existing devtools-frontend sub-repo from the Chromium checkout in chromium/src/.

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,

Then run

gclient sync -D

This removes the DevTools frontend dependency. We now create a symlink to refer to the standalone checkout (execute in chromium/src and make sure that third_party/devtools-frontend exists):

(Note that the folder names do NOT include the trailing slash)

Following this step, there are two approaches to integrating the standalone devtools.

Flavor 1: separate gclient projects

The first approach is to have separate gclient projects, one for each repository, and manually create a symlink. First, get a checkout of DevTools frontend.

To then create the symlink:

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

Running gclient sync in chromium/src/ will update dependencies for the Chromium checkout. Running gclient sync in chromium/src/third_party/devtools-frontend/src 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
    "url": "https://chromium.googlesource.com/chromium/src.git",
    "managed": False,
    "name": "src",
    "custom_deps": {
      "src/third_party/devtools-frontend/src": None,
    },
    "custom_vars": {},
  },
  {
    # devtools-frontend project
    "name": "devtools-frontend",
    "url": "https://chromium.googlesource.com/devtools/devtools-frontend",
    "custom_deps": {}
  }
]

Run gclient sync once in chromium/src/ to get the new devtools frontend checkout.

To automatically symlink between devtools-frontend and chromium/src, you can add the following hook to your .gclient file to manage your chromium/src repository after your list of solutions.

hooks = [
  {
    # Ensure devtools is symlinked in the correct location on every gclient sync
    'name': 'Symlink Depot Tools',
    'pattern': '.',
    'action': [
        'python',
        '<path>/<to>/devtools-frontend/scripts/deps/ensure_symlink.py',
        '<path>/<to>/chromium/src',
        '<path>/<to>/devtools-frontend'
    ],
  }
]

Running gclient sync anywhere within chromium/src/ or chromium/src/third_party/devtools-frontend/src will update dependencies for both checkouts. Running gclient sync -D will not remove your symlink.

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

Creating a change

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

Managing dependencies

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

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 The Zen of Merge Requests. In exceptional cases please get in touch with hablich@chromium.org.

Step-by-step guide on how to merge:

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.
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).
Open the to-be-merged commit in Gerrit (example).
Click the hamburger menu on the top right and select “Cherry pick”.
Select the branch to merge to e.g. chromium/3968.
The cherry-pick CL is created (example).
Get it reviewed if necessary.
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.)
Done.

Useful Commands

`git cl format --js`

Formats all code using clang-format.

`npm run check`

Runs all static analysis checks on DevTools code.