Get the Code: Checkout and Build Chromium DevTools front-end

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.

There are two tips to have a faster development workflow:

  • Disabling type checking for TypeScript.
  • Using watch script for faster incremental builds with CSS hot reload.

Disabling type checking

You can disable type checking for TypeScript by using devtools_skip_typecheck argument:

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

Faster incremental builds & CSS hot reload

In addition to that, you can enable CSS hot reload while using watch script with devtools_css_hot_reload_enabled argument:

gn gen out/fast-build --args="devtools_css_hot_reload_enabled=true"

with this and devtools_skip_typecheck=true in place, you can use watch script by:

npm run watch -- --target=fast-build

which will automatically apply CSS changes & instantly build changed TS files.

Caution! watch script is not based on ninja thus the build output and behavior might differ with the official ninja build. In addition to that, there might be some cases that watch build doesn't reliably handle, so you might need to run autoninja and restart the script from time to time.

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/MacOS/Google\ Chrome\ for\ Testing --disable-infobars --disable-features=MediaRouter --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 --disable-infobars --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 --disable-infobars --custom-devtools-frontend="<path-to-devtools-frontend>\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).

Tip: On Windows it is possible the browser will re-attach to an existing session without applying command arguments. Make sure that there are no active Chrome sessions running.

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/chrome/chrome-<platform>/chrome --disable-infobars --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 Chrome, allowing for accesses from the web server:

<path-to-devtools-frontend>/third_party/chrome/chrome-<platform>/chrome --disable-infobars --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.

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

Single gclient project

Note: it's not possible anymore to manage the two projects in separate gclient projects.

For the integrated checkout, create a single gclient project that automatically gclient sync's all dependencies for both repositories. After checking out chromium, modify the .gclient file for chromium/src to add the DevTools project:

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",
    "managed": False,
    "url": "https://chromium.googlesource.com/devtools/devtools-frontend.git",
  }
]

Do not run gclient sync now, first create the symlink. In the same directory as the .gclient file, run:

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

If you did run gclient sync first, remove the devtools-frontend directory and start over.

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