Thanks for being willing to contribute! 🙏

Working on your first Pull Request (PR)? You can learn how from this free series How to Contribute to an Open Source Project on GitHub.

Please check out the the open issues. Issues labelled Good First Issue are especially good to start with.

Contributing doesn’t have to be in code. Simply answering questions in open issues or providing workarounds is as important as making pull requests.

1. Install pnpm
2. Fork this repo and clone your copy locally
3. Run pnpm i to install dependencies

This project uses manual type declarations (docs) that are kept separately from the runtime code. index.d.ts contains type declarations, and index.js contains runtime code. The most important code lives in index.d.ts because this library exists to provide correct type inference. index.js, is far less important, and is only doing bare-minimum work to support the API as efficiently as possible.

In most projects, this is not recommended practice as the two can (and will) diverge, and you’re left with types that “lie” to you about what the runtime is doing. So this project does have that liability. However, due to the unique nature of this project implementing complex type inference from user-provided schemas, the type inference itself is so complex it was interfering with readable, maintainable, runtime code. By separating the two we won’t have the more complex parts of TypeScript interfering with the optimal runtime code. This would not be tenable in a larger project, but is perfect for a small codebase like this with minimal surface area.

When writing code, it’s tempting to ignore index.d.ts and only make runtime updates, since that is generally simpler. But please don’t! We write tests in *.test.ts files intentionally so that the type inference can be typechecked as well as the runtime. Whenever you make a change to index.js, you probably need to also make a chance to index.d.ts, too. And testing type correctness in tests is just as important as testing the runtime!

This library uses Vitest for testing. There’s a great VS Code extension you can optionally use if you’d like in-editor debugging tools.

To run the entire test suite, run:

pnpm test

To run an individual test:

pnpm test -- [partial filename]

To start the entire test suite in watch mode:

npx vitest

All tests in this project should adhere to the following rules:

1. Use assertType<T>(…) (docs). Don’t just check the actual runtime value; check the perceived type as well.
2. Testing TS errors is just as important as testing for expected types. Use // @ts-expected-error liberally. this is discouraged because it’s hiding an error. But in our tests, we want to test that a TS error is thrown for invalid input.
3. Scope // @ts-expect-error as closely as possible. Remember that JS largely ignores whitespace. When using // @ts-expect-error, try and break up an expression into as many lines as possible, so that the // @ts-expect-error is scoped to the right line. Otherwise it is possible a different part of the expression is throwing the error!
4. Manually type out type tests. Avoid using test.each for type tests, as it’s likely hiding errors.

Linting is handled via Biome, a faster ESLint replacement. It was installed with pnpm i and can be run with:

pnpm run lint

The changelog is generated via changesets, and is separate from Git commit messages and pull request titles. To write a human-readable changelog for your changes, run:

npx changeset

This will ask if it’s a patch, minor, or major change (semver), along with a plain description of what you did. Commit this new file along with the rest of your PR, and during the next release this will go into the official changelog!

Pull requests are welcome for this repo!

Bugfixes will always be accepted, though in some cases some small changes may be requested.

However, if adding a feature or breaking change, please open an issue first to discuss. This ensures no time or work is wasted writing code that won’t be accepted to the project (see Project Goals). Undiscussed feature work may be rejected at the discretion of the maintainers.

Create a new branch for your PR with git checkout -b your-branch-name. Add the relevant code as well as docs and tests. When you push everything up (git push), navigate back to your repo in GitHub and you should see a prompt to open a new PR.

While best practices for commit messages are encouraged (e.g. start with an imperative verb, keep it short, use the body if needed), this repo doesn’t follow any specific guidelines. Clarity is favored over strict rules. Changelogs are generated separately from git (see the Changelogs section).

Please fill out the template! It’s a very lightweight template 🙂.

If you added a feature, or changed how something worked, please update the docs!

All PRs must fix lint errors, and all tests must pass. PRs will not be merged until all CI checks are “green” (✅).