Gerrit Polymer Frontend

Follow the setup instructions for Gerrit backend developers where applicable, the most important command is:

git clone --recurse-submodules

The --recurse-submodules option is needed on git clone to ensure that the core plugins, which are included as git submodules, are also cloned.

Then make sure to install the commit-hook that will set up the ChangeId for each push to gerrit-reviews.

cd gerrit && (
  cd .git/hooks
  ln -s ../../resources/com/google/gerrit/server/tools/root/hooks/commit-msg

Installing Bazel

Follow the instructions here to get and install Bazel. The npm install -g @bazel/bazelisk method is probably easiest since you will have npm as part of Nodejs.

Installing Node.js and npm packages

At the time of writing (November 2023) you should use version 18 of nodejs.

# Debian experimental
sudo apt-get install nodejs
sudo apt-get install npm

# OS X with Homebrew
brew install node@18
brew install npm

All other platforms: download from

or use nvm - Node Version Manager.

Additional packages

We have several bazel commands to install packages we may need for FE development.

For first time users to get the local server up, bazel build gerrit should be enough and will take care of all of them for you.

# Install yarn package manager
npm install -g yarn

# Install packages from all packages.json files
yarn setup

More information for installing and using nodejs rules can be found here

Upgrade to @bazel-scoped packages

It might be necessary to run this command to upgrade to major rules_nodejs release:

yarn remove @bazel/...

Setup typescript support in the IDE

Modern IDEs should automatically handle typescript settings from the polygerrit-ui/app/tsconfig.json files. The tsc compiler places compiled files in the .ts-out/pg directory at the root of gerrit workspace and you can configure the IDE to exclude the whole .ts-out directory. To do it in the IntelliJ IDEA click on this directory and select “Mark Directory As > Excluded” in the context menu.

However, if you receive some errors from IDE, you can try to configure IDE manually. For example, if IntelliJ IDEA shows Cannot find parent 'tsconfig.json' error, you can try to setup typescript options --project polygerrit-ui/app/tsconfig.json in the IDE settings.

Developing locally

The preferred method for development is to serve the web files locally using the Web Dev Server and then view a running gerrit instance (local or otherwise) to replace its web client with the local one using the Gerrit FE Dev Helper extension.

Web Dev Server

The Web Dev Server serves the compiled web files and dependencies unbundled over localhost. Start it using this command:

yarn start

To inject plugins or other files, we use the Gerrit FE Dev Helper Chrome extension.

If any issues occured, please refer to the Troubleshooting section at the bottom or contact the team!

Chrome extension: Gerrit FE Dev Helper

To be able to bypass the auth and also help improve the productivity of Gerrit FE developers, we created this chrome extension: Gerrit FE Dev Helper.

It basically works as a proxy that will block / redirect requests from current sites to any given url base on certain rules.

The source code is in Gerrit - gerrit-fe-dev-helper, contributions are welcomed!

To use this extension, just follow its readme here.

Running locally against a Gerrit test site

Set up a local test site once:

  1. Build Gerrit
  2. Set up a local test site.
  3. Optionally populate your test site with some test data.

For running a locally built Gerrit war against your test instance use this command.

If you want to serve the Polymer frontend directly from the sources in polygerrit_ui/app/ instead of from the war:

  1. Start Web Dev Server
  2. Add the --dev-cdn option:
$(bazel info output_base)/external/local_jdk/bin/java \
    -DsourceRoot=$(bazel info workspace) \
    -jar bazel-bin/gerrit.war daemon \
    -d $GERRIT_SITE \
    --console-log \
    --dev-cdn http://localhost:8081

The Web Dev Server is currently not serving fonts or other static assets. Follow Issue 40015119 for fixing this issue.

NOTE You can use any other cdn here, for example:

Running Tests

For daily development you typically only want to run and debug individual tests. Our tests run using the Web Test Runner. There are several ways to trigger tests:

  • Run all tests once:
yarn test
  • Run all tests and then watches for changes. Change a file will trigger all tests affected by the changes.
yarn test:watch
  • Run all tests once under bazel:
  • Run a single test file and rerun on any changes affecting it:
yarn test:single "**/gr-comment_test.ts"

Compiling code:

# Compile frontend once to check for type errors:
yarn compile

# Watch mode:
yarn compile:watch

Style guide

We follow the Google JavaScript Style Guide with a few exceptions. When in doubt, remain consistent with the code around you.

In addition, we encourage the use of ESLint. It is available as a command line utility, as well as a plugin for most editors and IDEs.

eslint-config-google is a port of the Google JS Style Guide to an ESLint config module, and eslint-plugin-html allows ESLint to lint scripts inside HTML. We have an .eslintrc.json config file in the polygerrit-ui/ directory configured to enforce the preferred style of the PolyGerrit project. After installing, you can use eslint on any new file you create. In addition, you can supply the --fix flag to apply some suggested fixes for simple style issues. If you modify JS inside of <script> tags, like for test suites, you may have to supply the --ext .html flag.

Some useful commands:

  • To run ESLint on the whole app, less some dependency code:
yarn eslint
  • To run ESLint on just the subdirectory you modified:
node_modules/eslint/bin/eslint.js --ext .html,.js polygerrit-ui/app/$YOUR_DIR_HERE
  • To run the linter on all of your local changes:
git diff --name-only HEAD | xargs node_modules/eslint/bin/eslint.js --ext .html,.js

Migrating tests to Typescript

You can use the following steps for migrating tests to Typescript:

  1. Rename the _test.js file to _test.ts
  2. Remove .js extensions from all imports:
    // Before:
    import ... from 'x/y/z.js`
    // After
    import .. from 'x/y/z'
  3. Fix typescript and eslint errors.

Common errors and fixes are:

  • An object in the test doesn't have all required properties. You can use existing helpers to create an object with all required properties:
// Before:
sinon.stub(element.restApiService, 'getPreferences').returns(
    Promise.resolve({default_diff_view: 'UNIFIED'}));

// After:
  default_diff_view: DiffViewMode.UNIFIED,

Some helpers receive parameters:

// Before
element._change = {
  change_id: 'Iad9dc96274af6946f3632be53b106ef80f7ba6ca',
  revisions: {
    rev1: {_number: 1, commit: {parents: []}},
    rev2: {_number: 2, commit: {parents: []}},
  current_revision: 'rev1',
  status: ChangeStatus.MERGED,
  labels: {},
  actions: {},

// After
element._change = {
  // The change_id is set by createChange.
  // The exact change_id is not important in the test, so it was removed.
  revisions: {
    rev1: createRevision(1), // _number is a parameter here
    rev2: createRevision(2), // _number is a parameter here
  current_revision: 'rev1' as CommitId,
  status: ChangeStatus.MERGED,
  labels: {},
  actions: {},
  • Typescript reports some weird messages about window property - sometimes an IDE adds wrong import. Just remove it.
// The wrong import added by IDE, must be removed
import window = Mocha.reporters.Base.window;
  • TS2531: Object is possibly 'null'. To fix use either non-null assertion operator ! or nullish coalescing operator ?.:
// Before:
const rows = element
  .querySelectorAll('tbody tr');
// The _robotCommentThreads declared as _robotCommentThreads?: CommentThread
assert.equal(element._robotCommentThreads.length, 2);

// Fix with non-null assertion operator:
const rows = element
  .shadowRoot!.querySelector('table')! // '!' after shadowRoot and querySelector
  .querySelectorAll('tbody tr');

assert.equal(element._robotCommentThreads!.length, 2);

// Fix with nullish coalescing operator:
 assert.equal(element._robotCommentThreads?.length, 2);

Usually the fix with ! is preferable, because it gives more clear error when an intermediate property is null/undefined. If the _robotComments is undefined in the example above, the element._robotCommentThreads!.length crashes with the error Cannot read property 'length' of undefined. At the same time the fix with ?. doesn't distinct between 2 cases: _robotCommentThreads is undefined and length is undefined.

  • TS2339: Property '...' does not exist on type 'Element'. for elements returned by querySelector/querySelectorAll. To fix it, use generic versions of those methods:
// Before:
const radios = parentTable
const radio = parentRow

// After:
const radios = parentTable
const radio = parentRow
  • Sinon: TS2339: Property 'lastCall' does not exist on type '... (the same for other sinon properties). Store stub/spy in a variable and then use the variable:
// Before:
sinon.stub(GerritNav, 'getUrlForChange')
assert.equal(GerritNav.getUrlForChange.lastCall.args[4], '#message-a12345');

// After:
const getUrlStub = sinon.stub(GerritNav, 'getUrlForChange');
assert.equal(getUrlStub.lastCall.args[4], '#message-a12345');

If you need to define a type for such variable, you can use one of the following options:

suite('my suite', () => {
    // Non static members, option 1
    let updateHeightSpy: SinonSpyMember<typeof element._updateRelatedChangeMaxHeight>;
    // Non static members, option 2
    let updateHeightSpy_prototype: SinonSpyMember<typeof GrChangeView.prototype._updateRelatedChangeMaxHeight>;
    // Static members
    let navigateToChangeStub: SinonStubbedMember<typeof GerritNav.navigateToChange>;
    // For interfaces
    let getMergeableStub: SinonStubbedMember<RestApiService['getMergeable']>;
  • Typescript reports errors when stubbing/faking methods:
// The JS code:
const reloadStub = sinon
    .stub(element, '_reload')
    .callsFake(() => Promise.resolve());


In such cases, validate the input and output of a stub/fake method. Quite often tests return null instead of undefined or [] instead of {}, etc... Fix types if they are not correct:

const reloadStub = sinon
  .stub(element, '_reload')
  // GrChangeView._reload method returns an array
  .callsFake(() => Promise.resolve([])); // return [] here

  // Fix return type:
  stubRestApi('_fetchSharedCacheURL').returns(Promise.resolve({} as ParsedJSON));
  • If a test requires a @types/... library, install the required library in the polygerrit_ui/node_modules and update the typeRoots in the polygerrit-ui/app/tsconfig_bazel_test.json file.

The same update should be done if a test requires a .d.ts file from a library that already exists in polygerrit_ui/node_modules.

Note: Types from a library located in polygerrit_ui/app/node_modules are handle automatically.

  • If a test imports a library from polygerrit_ui/node_modules - update paths in polygerrit-ui/app/tsconfig_bazel_test.json.


Our users report bugs / feature requests related to the UI through the Gerrit Tracker on the WebFrontend component.

If you want to help, feel free to grab one from those New issues without assignees and send us a change.

If you don't know who to assign to review your code change, you can use this special account: and just assign to that account, it will automatically pick two volunteers from the queue we have for FE reviewers.

If you are willing to join the queue and help the community review changes, you can create an issue through Monorail and request to join the queue! We will review your request and start from there.

Troubleshotting & Frequently asked questions

  1. Local host is blank page and console shows missing files from polymer-bridges

Its likely you missed the polymer-bridges submodule when you clone the gerrit repo.

To fix that, run:

// fetch the submodule
git submodule update --init --recursive

// reset the workspace (please save your local changes before running this command)
yarn clean

// install all dependencies and start the server
npm start