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

• The polyfill implementation should be added to the packages/core-js/modules directory.
• The polyfill should properly work in ES3 and all possible engines. If in some engines it cannot be implemented (for example, it strictly requires more modern ES or unavailable platform features), it should not break any other core-js features or application in any way.
• Avoid possible observing / breakage polyfills via patching built-ins at runtime: cache all global built-ins in the polyfills code and don't call prototype methods from instances.
• Shared helpers should be added to the packages/core-js/internals directory. Reuse already existing helpers.
• Avoid direct import from /modules/ path in /internals|modules/ since it will break optimizations via Babel / swc. Specify such dependencies in /es|stable|actual/full/ entries and use something like internals/get-built-in helpers.
• For export the polyfill, in all common cases use internals/export helper. Use something else only if this helper is not applicable - for example, if you want to polyfill accessors.
• If the code of the pure version implementation should significantly differ from the global version (that's not a frequent situation, in most cases internals/is-pure constant is enough), you can add it to packages/core-js-pure/override directory. The rest parts of core-js-pure will be copied from core-js package.
• Add the feature detection of the polyfill to tests/compat/tests.js, add the compatibility data to packages/core-js-compat/src/data.mjs, how to do it see below, and the name of the polyfill module to packages/core-js-compat/src/modules-by-versions.mjs (this data is also used for getting the default list of polyfills at bundling and generation indexes).
• Add it to entry points where it's required: directories packages/core-js/es, packages/core-js/stable, packages/core-js/actual, packages/core-js/full, packages/core-js/proposals, packages/core-js/stage and packages/core-js/web.
• Add unit tests to tests/unit-global and tests/unit-pure.
• Add tests of entry points to tests/entries/unit.mjs.
• Make sure that you are following our coding style and all tests are passed.
• Document it in site documentation and CHANGELOG.md.

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/master/compat) and you will see what core-js modules are required for this browser.

• 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, run npm run compat-rhino YOUR_PATH_TO_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 YOUR_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 - 16)
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 prepare monorepo and install dependencies:

npm run prepare-monorepo

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