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Contributing to Lightning Web Components

We want to encourage the developer community to contribute to Lightning Web Components. This guide has instructions to install, build, test and contribute to the framework.

Before you start, familiarize yourself with Lightning Web Components.

Requirements

This project uses Volta to ensure that all the contributors share the same version of Node and Yarn for development. If you are considering making frequent contributions to this project, we recommend installing this tool as well. Otherwise, check the volta field in package.json to see which versions to use.

Installation

Set up SSH access to Github if you haven't done so already.

1) Download the repository

git clone git@github.com:salesforce/lwc.git

2) Install Dependencies

We use yarn because it is significantly faster than npm for our use case. See this command cheatsheet.

yarn install

If this fails with an error about UNABLE_TO_GET_ISSUER_CERT_LOCALLY, Error: unable to get local issuer certificate, or a registry communication issue then re-verify that step 2 was successful.

Building LWC

yarn build

Dev / watch mode

yarn dev

Testing

Unit Testing LWC

When developing LWC, utilize jest unit testing to provide test coverage for new functionality. To run the jest tests use the following command from the root directory:

yarn test

Additionally, the testing can be started in 'watch' mode which allows for automatic test re-runs on save:

yarn test --watch

To execute a particular test, use the following command:

yarn test <path_to_test>

If you change the way the compiler outputs code, then you may see failed tests due to these changes. In those cases, you can regenerate the test snapshots like so:

yarn test -u

If you want to debug these tests, you can do as follow:

  1. First, insert a new line in your test where you think it might be failing and type debugger. This will serve as a break point for the debugger to stop at.
  2. Open up Chrome and type in the address bar: chrome://inspect
  3. Click on "Open dedicated DevTools for Node"
  4. In your terminal, type the following command: yarn test:debug <path_to_test>

Your test should now be running in the Chrome debugger which you can use to poke around and explore. Now simply hit Enter in the terminal running your Jest process anytime you want to re-run your currently selected specs. You'll be dropped right back into the Chrome debugger.

Integration Testing LWC

When developing LWC, use integration testing to ensure functionality is correctly reflected in the browser. This repo has two integration test suites.

  • @lwc/integration-karma: Contains all integration tests that can run with javascript only. For information about usage and contribution, refer to this documentation.
  • @lwc/integration-tests: Contains all other integration tests that require web driver API support (e.g., focus, keyboard navigation). For information about usage and contribution, refer to this documentation.

Performance testing LWC

yarn build
yarn build:performance
yarn test:performance

This will run all performance tests comparing the current code to the latest master branch. See the @lwc/perf-benchmarks package's README for more details.

Bundle size monitoring

yarn build
yarn bundlesize

This will check that the minified bundle sizes don't exceed our thresholds. Our goal is to stay consciously aware of the bundle sizes as the repository grows over time.

Types

LWC relies on type annotations.

ESLint

Configure your editor to use our eslint configurations.

Git Workflow

The process of submitting a pull request is fairly straightforward and generally follows the same pattern each time:

  1. Fork the LWC repo
  2. Create a feature branch
  3. Make your changes
  4. Rebase
  5. Check your submission
  6. Create a pull request
  7. Update the pull request
  8. Commit Message Guidelines

Fork the LWC repo

Fork the salesforce/lwc repo. Clone your fork in your local workspace and configure your remote repository settings.

git clone git@github.com:<YOUR-USERNAME>/lwc.git
cd lwc
git remote add upstream git@github.com:salesforce/lwc.git

Create a feature branch

git checkout master
git pull origin master
git checkout -b <name-of-the-feature>

Make your changes

Modify the files, build, test, lint and eventually commit your code using the following command:

git add <path/to/file/to/commit>
git commit or git cz
git push origin <name-of-the-feature>

Commit your changes using a descriptive commit message that follows our Commit Message Guidelines. Adherence to these conventions is necessary because release notes are automatically generated from these messages. NOTE: optional use of git cz command triggers interactive semantic commit, which prompts user with commit related questions, such as commit type, scope, description, and breaking changes. Use of git cz is optional but recommended to ensure format consistency.

The above commands will commit the files into your feature branch. You can keep pushing new changes into the same branch until you are ready to create a pull request.

Rebase

Sometimes your feature branch will get stale with respect to the master branch, and it will require a rebase. The following steps can help:

git checkout master
git pull upstream master
git checkout <name-of-the-feature>
git rebase upstream/master

note: If no conflicts arise, these commands will ensure that your changes are applied on top of the master branch. Any conflicts will have to be manually resolved.

Check your submission

Lint your changes

yarn run lint

The above command may display lint issues that are unrelated to your changes. The recommended way to avoid lint issues is to configure your editor to warn you in real time as you edit the file.

Fixing all existing lint issues is a tedious task so please pitch in by fixing the ones related to the files you make changes to!

Run tests

Test your change by running the unit tests and integration tests. Instructions here.

Create a pull request

If you've never created a pull request before, follow these instructions. Pull request samples can be found here

Pull Request Title

A pull request title follows conventional commit format and is automatically validated by our CI.

ex:
commit-type(optional scope): commit description. ( NOTE: space between column and the message )

Types: build, chore, ci, docs, feat, fix, perf, refactor, revert, style, test, proposal.
Scope: The scope should be the name of the npm package affected (engine, compiler, wire-service, etc.)

Update the pull request

git fetch origin
git rebase origin/${base_branch}

# If there were no merge conflicts in the rebase
git push origin ${feature_branch}

# If there was a merge conflict that was resolved
git push origin ${feature_branch} --force

note: If more changes are needed as part of the pull request, just keep committing and pushing your feature branch as described above and the pull request will automatically update.

Commit Message Conventions

Git commit messages have to be formatted according to a well defined set of rules. This leads to more readable messages that are easy to follow when looking through the project history.

Commit Message Format

Each commit message consists of a header, a body and a footer. The header has a special format that includes a type, a scope and a subject:

<type>(<scope>): <subject>
<BLANK LINE>
<body>
<BLANK LINE>
<footer>

The header is mandatory and the scope of the header is optional.

Any line of the commit message cannot be longer 100 characters! This allows the message to be easier to read on GitHub as well as in various git tools.

Footer should contain a closing reference to an issue if any.

Samples: (even more samples)

docs(changelog): update change log to beta.5

fix(release): need to depend on latest rxjs and zone.js

The version in our package.json gets copied to the one we publish, and users need the latest of these.

Reverting a commit

If the commit reverts a previous commit, it should begin with revert: , followed by the header of the reverted commit. In the body it should say: This reverts commit <hash>., where the hash is the SHA of the commit being reverted.

Commit Type

Must be one of the following:

  • build: Changes that affect the build system or external dependencies (example scopes: gulp, broccoli, npm)
  • chore: Other changes that don't modify src or test files
  • ci: Changes to our CI configuration files and scripts (example scopes: Travis, Circle, BrowserStack, SauceLabs)
  • docs: Documentation only changes
  • feat: A new feature
  • fix: A bug fix
  • perf: A code change that improves performance
  • refactor: A code change that neither fixes a bug nor adds a feature
  • revert: Reverts a previous commit
  • style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
  • test: Adding missing tests or correcting existing tests

Commit Scope

The scope should be the name of the npm package affected, as perceived by the person reading the changelog.

There are currently a few exceptions to the "use package name" rule:

  • packaging: used for changes that change the npm package layout in all of our packages, e.g. public path changes, package.json changes done to all packages, d.ts file/format changes, changes to bundles, etc.
  • changelog: used for updating the release notes in CHANGELOG.md
  • lwc docs: used for docs related changes within the lwc/docs directory of the repo
  • none/empty string: useful for style, test and refactor changes that are done across all packages (e.g. style: add missing semicolons)

Commit Subject

The subject contains a succinct description of the change:

  • use the imperative, present tense: "change" not "changed" nor "changes"
  • don't capitalize first letter
  • no dot (.) at the end

Commit Body

Just as in the subject, use the imperative, present tense: "change" not "changed" nor "changes". The body should include the motivation for the change and contrast this with previous behavior.

Commit Footer

The footer should contain any information about Breaking Changes and is also the place to reference GitHub issues that this commit closes.

Breaking Changes should start with the word BREAKING CHANGE: with a space or two newlines. The rest of the commit message is then used for this.

Getting your changes reviewed

Potential pitfalls: When submitting changes to the LWC framework, there are several important considerations to keep in mind to ensure that your changes are reviewed & accepted.

  • The LWC framework makes strong guarantees about backwards compatibility to those that use our framework. Any changes that compromise these guarantees are unlikely to be accepted as-is.
  • If your change causes a test to fail in unit or integration tests, it is unlikely that your submission will be accepted as-is.
  • Certain parts of the codebase are particularly susceptible to breaking changes. This includes the browser runtime (engine-core and engine-dom), as well as the compiler (compiler, style-compiler, template-compiler).
  • Some areas of the codebase are less mature and undergoing active development. Areas related to SSR, including the server-side runtime (engine-server), fall under this category.

Definition of done: Especially when making large changes to the codebase, certain complementary work must be undertaken for the desired changes to be considered. When making a submission please adhere to the following:

  • All pre-existing unit- & integration- tests must pass.
  • If you modify a pre-existing test to accommodate your changes, a thorough reasoning must be proactively provided in the PR.
  • Test coverage should not decline – if you add code paths, add corresponding tests.
  • If your tests have the potential to negatively impact performance, provide a before & after comparison of performance using the test collateral provided in the repo.

It's also worth noting that not all submissions will be accepted, even if all the above criteria are met. There may be constraints in the larger Salesforce ecosystem that prevent certain changes. If your submission is likely to change large parts of the codebase, please open an issue preemptively so that the design can be discussed.