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For Contributors

We'd love your help! This doc covers how to become a contributor and submit code to the project.

Follow the coding style

The .eslintrc file defines all. We use JSDoc with TypeScript. Annotations are encouraged for all contributions.

Pull request titles

We're using conventional-commit for our commit messages. Since all PRs are squashed, we enforce this format for PR titles rather than individual git commits. A commitlint bot will update the status of your PR based on the title's conformance. The expected format is:

type(scope): message subject

  • The type must be one of: new_audit core tests docs deps report cli extension misc. (See .cz-config)
  • The scope is optional, but recommended. Any string is allowed; it should indicate what the change affects.
  • The message subject should be pithy and direct.

The commitizen CLI can help to construct these commit messages.

Learn about the architecture

See Lighthouse Architecture, our overview and tour of the codebase.

Sign the Contributor License Agreement

We'd love to accept your sample apps and patches! Before we can take them, we have to jump a couple of legal hurdles.

Please fill out either the individual or corporate Contributor License Agreement (CLA).

  • If you are an individual writing original source code and you're sure you own the intellectual property, then you'll need to sign an individual CLA.
  • If you work for a company that wants to allow you to contribute your work, then you'll need to sign a corporate CLA.

Follow either of the two links above to access the appropriate CLA and instructions for how to sign and return it. Once we receive it, we'll be able to accept your pull requests.

Contributing a patch

If you have a contribution for our documentation, please submit it in the WebFundamentals repo.

  1. Submit an issue describing your proposed change to the repo in question.
  2. The repo owner will respond to your issue promptly.
  3. If your proposed change is accepted, and you haven't already done so, sign a Contributor License Agreement (see details above).
  4. Fork the repo, develop and test your code changes.
  5. Ensure that your code adheres to the existing style in the sample to which you are contributing.
  6. Submit a pull request.

Adding Images to a Readme

If you are adding an image to a readme use the absolute path to the image for the specific commit hash where the image was introduced. This requires mutliple commits.

  1. Make the commit to introduce the image.
  2. Get the absolute path to the image with the commit hash e.g.
  3. Add to readme as an absolute reference to that image.

If you are updating an image that already exists: commit it, then update the readme to point the image with that new commits hash absolute url.

Description Guidelines

Keep the description of an audit as short as possible. When a reference doc for the audit exists on, the description should only explain why the user should care about the audit, not how to fix it.


Serve images that are smaller than the user's viewport to save cellular data and
improve load time. [Learn more](


Serve images that are smaller than the user's viewport to save cellular data and
improve load time. Consider using responsive images and client hints.

If no reference doc exists yet, then you can use the description as a stopgap for explaining both why the audit is important and how to fix it.

Tracking Errors

We track our errors in the wild with Sentry. In general, do not worry about wrapping your audits or gatherers in try/catch blocks and reporting every error that could possibly occur; lighthouse-core/runner.js and lighthouse-core/gather/gather-runner.js already catch and report any errors that occur while running a gatherer or audit, including errors fatal to the entire run. However, there are some situations when you might want to expliticly handle an error and report it to Sentry or wrap it to avoid reporting. Generally, you can interact with Sentry simply by requiring the lighthouse-core/lib/sentry.js file and call its methods. The module exports a delegate that will correctly handle the error reporting based on the user's opt-in preference and will simply no-op if they haven't so you don't need to check.

If you have an expected error that is recoverable but want to track how frequently it happens, use Sentry.captureException.

const Sentry = require('./lighthouse-core/lib/sentry');

try {
} catch (err) {
  Sentry.captureException(err, {
    tags: {audit: 'audit-name'},
    level: 'warning',

If you need to track a code path that doesn't necessarily mean an error occurred, use Sentry.captureMessage.

NOTE: If the message you're capturing is dynamic/based on user data or you need a stack trace, then create a fake error instead and use Sentry.captureException so that the instances will be grouped together in Sentry.

const Sentry = require('./lighthouse-core/lib/sentry');

if (networkRecords.length === 1) {
  Sentry.captureMessage('Site only had 1 network request', {level: 'info'});
  return null;
} else {
  // do your thang

Level Guide

  • info for events that don't indicate a bug but should be tracked
  • warning for events that might indicate unexpected behavior but is recoverable
  • error for events that caused an audit/gatherer failure but were not fatal
  • fatal for events that caused Lighthouse to exit early and not produce a report

For Maintainers

The release guide.