# For Contributors

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

## Where can I start?

We tag issues that are good candidates for those new to the code with [`good first issue`](https://github.com/GoogleChrome/lighthouse/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+label%3A%22good+first+issue%22). We recommend you start there!

## Follow the coding style

The `.eslintrc` file defines all. We use [JSDoc](http://usejsdoc.org/) with [TypeScript](https://github.com/Microsoft/TypeScript/wiki/JSDoc-support-in-JavaScript). Annotations are encouraged for all contributions.

## Pull request titles

We're using [conventional-commit](https://conventionalcommits.org/) for our commit messages. Since all PRs are squashed, we enforce this format for PR titles rather than individual git commits. A [`commitlint` bot](https://github.com/paulirish/commitlintbot) 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`](https://github.com/GoogleChrome/lighthouse/blob/master/.cz-config.js#L13))
* 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](https://github.com/commitizen/cz-cli) can help to construct these commit messages.

## Learn about the architecture

See [Lighthouse Architecture](./docs/architecture.md), 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](https://developers.google.com/open-source/cla/individual).
* If you work for a company that wants to allow you to contribute your work, then you'll need to sign a [corporate CLA](https://developers.google.com/open-source/cla/corporate).

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](https://developers.google.com/web/tools/lighthouse/), please submit it in the [WebFundamentals repo](https://github.com/google/WebFundamentals/tree/master/src/content/en/tools/lighthouse).

1. Submit an issue describing your proposed change to the repo in question.
1. The repo owner will respond to your issue promptly.
1. If your proposed change is accepted, and you haven't already done so, sign a Contributor License Agreement (see details above).
1. Fork the repo, develop and test your code changes.
1. Ensure that your code adheres to the existing style in the sample to which you are contributing.
1. 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 multiple commits.
1. Make the commit to introduce the image.
1. Get the [absolute path](https://help.github.com/articles/getting-permanent-links-to-files/) to the image with the commit hash e.g. `https://raw.githubusercontent.com/GoogleChrome/lighthouse/e7997b3db01de3553d8cb208a40f3d4fd350195c/assets/example_dev_tools.png`
1. 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
developers.google.com/web, the `description` should only explain *why* the user should care
about the audit, not *how* to fix it.

Do:

    Serve images that are smaller than the user's viewport to save cellular data and
    improve load time. [Learn more](https://developers.google.com/web/tools/lighthouse/audits/oversized-images).

Don't:

    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.

## Updating sample artifacts and LHR JSON

```
yarn run update:sample-artifacts # update all artifacts
yarn run update:sample-artifacts ScriptElements # update just one artifact
yarn run update:sample-json # update sample LHR based on sample artifacts
```

When updating artifacts, usually you'll need to revert changes to the `*.devtoolslog.json` and `*.trace.json` files and manually review changes to `artifacts.json` to make sure they are related to your work.

## 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 explicitly 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*.

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

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

#### 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.

```js
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](./docs/releasing.md).