CONTRIBUTING.md

Contributing

Contributions are always welcome. If you don't know how you can help, you can check issues or ask @zloirock.

How to add a new polyfill

• The polyfill implementation should be added to the packages/core-js/modules directory.
• Shared helpers should be added to the packages/core-js/internals directory. Reuse already existing 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 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/tests and tests/pure.
• Add tests of entry points to tests/commonjs.js.
• Make sure that you are following our coding style and all tests are passed.
• Document it in README.md and CHANGELOG.md.

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.

• 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 Rhino, set the required Rhino version in compat-rhino-prepare NPM script in package.json, run npm run compat-rhino 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	mandatory check	how to run tests	base data inherits from	mapping for a new version
android		browser runner	chrome
chrome	features	browser runner
deno	non-ES features	deno runner	chrome (only ES)	required
edge	features	browser runner	ie (<=18), chrome (>=74)
electron		browser runner	chrome	required
firefox	features	browser runner
ie	features	browser runner
ios		browser runner	safari	required
node	non-ES features	node runner	chrome (only ES)	required
oculus		browser runner	android (-> chrome)	required
opera		browser runner	chrome (>12)	in case of inconsistency
opera_mobile		browser runner	opera, chrome	required
phantom		browser runner	safari
rhino	features	rhino runner
safari	features	browser runner
samsung		browser runner	chrome	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 .eslintrc. 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 all tests by

$ npm run test

You can run parts of the test case separately:

• Linting:

  $ npm run lint
  

• The global version unit tests:

  $ npm run test-unit-global-standalone
  

• The pure version unit tests:

  $ npm run test-unit-pure-standalone
  

• Promises/A+ and ES6 Promise test cases:

  $ npm run test-promises-standalone
  

• ECMAScript Observable test case:

  $ npm run test-observables-standalone
  

• CommonJS entry points tests:

  $ npm run test-entries-standalone
  

• If you want to run tests in a certain browser, at first, you should build packages and test bundles:

  $ npm run bundle-standalone
  

• For running the global version of the unit test case, use this file:

  tests/tests.html
  

• For running the pure version of the unit test case, use this file:

  tests/pure.html