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CONTRIBUTING.md

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Contributing

Contributions are always welcome. Feel free to ask @zloirock if you have some questions.

I want to help with code, but I don't know how

There is always some "help wanted" issues. You can look at them first. Sure, other help is also required - you could ask @zloirock about it or open issues if you have some ideas.

How to add a new polyfill

A simple example of adding a new polyfill.

How to update core-js-compat data

For updating core-js-compat data:

  • If you want to add a new data for a browser, run in this browser tests/compat/index.html (tests and results for the actual release are available at http://zloirock.github.io/core-js/compat/) and you will see what core-js modules are required for this browser.

compat-table

  • If you want to add new data for NodeJS, run npm run compat-node with the installed required NodeJS version and you will see the results in the console. Use npm run compat-node json if you want to get the result as JSON.
  • If you want to add new data for Deno, run npm run compat-deno with the installed required Deno version and you will see the results in the console. Use npm run compat-deno json if you want to get the result as JSON.
  • If you want to add new data for Bun, run npm run compat-bun with the installed required Bun version and you will see the results in the console.
  • If you want to add new data for Rhino, set the required Rhino version in compat-rhino NPM script in package.json, run npm run compat-rhino and you will see the results in the console.
  • If you want to add new data for Hermes (incl. shipped with React Native), run npm run compat-hermes YOR_PATH_TO_HERMES and you will see the results in the console.
  • After getting this data, add it to packages/core-js-compat/src/data.mjs.
  • If you want to add new mapping (for example, to add a new iOS Safari version based on Safari or NodeJS based on Chrome), add it to packages/core-js-compat/src/mapping.mjs.
engine how to run tests base data inherits from mandatory check mapping for a new version
android browser runner chrome, chrome-android
bun bun runner safari (only ES) required
chrome browser runner required
chrome-android browser runner chrome
deno deno runner chrome (only ES) non-ES features required
edge browser runner ie, chrome required (<= 18)
electron browser runner chrome required
firefox browser runner required
firefox-android browser runner firefox
hermes hermes runner required
ie browser runner required
ios browser runner safari if inconsistent (!= safari)
node node runner chrome (only ES) non-ES features required
opera browser runner chrome if inconsistent (!= chrome - 14)
opera-android browser runner opera, chrome-android required
phantom browser runner safari
quest browser runner chrome-android required
react-native hermes runner hermes required
rhino rhino runner required
safari browser runner required
samsung browser runner chrome-android required

If you have no access to all required browsers / versions of browsers, use Sauce Labs, BrowserStack or Cloud Browser.

Style and standards

The coding style should follow our eslint.config.js. You can test it by calling npm run lint. Different places have different syntax and standard library limitations:

  • Polyfill implementations should use only ES3 syntax and standard library, they should not use other polyfills from the global scope.
  • Unit tests should use the modern syntax with our minimalistic Babel config. Unit tests for the pure version should not use any modern standard library features.
  • Tools, scripts and tests, performed in NodeJS, should use only the syntax and the standard library available in NodeJS 8.

File names should be in the kebab-case. Name of polyfill modules should follow the naming convention namespace.subnamespace-where-required.feature-name, for example, esnext.set.intersection. The top-level namespace should be es for stable ECMAScript features, esnext for ECMAScript proposals and web for other web standards.

Testing

Before testing, you should install dependencies:

npm i

You can run the most tests by

npm t

You can run parts of the test case separately:

  • Linting:
    npm run lint
  • Unit test case in Karma (modern Chromium, Firefox, WebKit (Playwright), ancient WebKit (PhantomJS), IE11 (if available)):
    npx run-s prepare bundle test-unit-karma
  • Unit test case in NodeJS:
    npx run-s prepare bundle test-unit-node
  • Unit test case in Bun:
    npx run-s prepare bundle test-unit-bun
  • Test262 test case (it's not included to the default tests):
    npx run-s prepare bundle-package test262
  • Promises/A+ and ES6 Promise test cases:
    npx run-s prepare test-promises
  • ECMAScript Observable test case:
    npx run-s prepare test-observables
  • CommonJS entry points tests:
    npx run-s prepare test-entries
  • core-js-compat tools tests:
    npx run-s prepare test-compat-tools
  • core-js-builder tests:
    npx run-s prepare test-builder
  • If you want to run tests in a certain browser, at first, you should build packages and test bundles:
    npx run-s prepare bundle
  • For running the global version of the unit test case, use this file:
    tests/unit-browser/global.html
  • For running the pure version of the unit test case, use this file:
    tests/unit-browser/pure.html