core-js-checks

Preserve core-js compatibility checks with Rollup

Update, 2026-10-03: Rollup 4.64.0 includes the upstream correction in PR #6541, and issue #6538 is closed. Prefer upgrading and checking your final bundle. The temporary helper below is for projects that still use the affected older version.

Separate verification on 4.64.0, CommonJS plugin 29.0.3, node-resolve 16.0.3 and core-js 3.50.0 passed all four checks without the helper: actual primitive detections ran, repeated primitive/object operations completed, unused application code was removed, and a cached rebuild remained correct. The older-native case is a Node VM simulation, not a real older browser.

To rerun that verification from the current source directory in a disposable checkout:

npm ci --ignore-scripts
npm install --ignore-scripts --save-exact rollup@4.64.0
node --test test-current.mjs

The original npm test and versioned ZIP remain pinned to 4.63.5 and include a test that expects the old defect. Use test-current.mjs for the upgrade check. Remove the temporary helper from your own configuration only after testing your supported browsers and any downstream minifier.

Older-version workaround

Rollup 4.63.5 can remove calls inside a feature-detection callback passed through a property, even with the default tryCatchDeoptimization: true. In the tested CommonJS build of core-js 3.50.0, this drops the primitive checks for Object.isExtensible, Object.isFrozen and Object.isSealed. An engine that throws for these primitive arguments can then keep its incompatible native implementation.

This temporary configuration keeps tree-shaking enabled for your application and disables it for the core-js modules included in the build. It uses Rollup's documented module flag; it does not patch Rollup or core-js. See the original report.

Add the configuration

Copy keep-core-js-checks.mjs beside your Rollup configuration, then put it after the CommonJS plugin:

import commonjs from '@rollup/plugin-commonjs';
import { nodeResolve } from '@rollup/plugin-node-resolve';
import { keepCoreJsChecks } from './keep-core-js-checks.mjs';

export default {
  input: 'src/index.js',
  plugins: [nodeResolve(), commonjs(), keepCoreJsChecks()],
  output: { file: 'dist/index.js', format: 'es' },
};

The entire configuration helper is:

export function keepCoreJsChecks() {
  return {
    name: 'keep-core-js-checks',
    transform(code, id) {
      if (/[/\\]node_modules[/\\]core-js[/\\]/.test(id)) {
        return { code, map: null, moduleSideEffects: 'no-treeshake' };
      }
      return null;
    },
  };
}

moduleSideEffects: true alone still allows statements within a module to be removed. The string 'no-treeshake' is the relevant plugin hook result. Do not return this string from the global treeshake.moduleSideEffects callback, whose documented result is boolean.

Keep your existing core-js imports. This does not load additional polyfills for you. Matching follows the resolved module ID: a node_modules/core-js/ path segment is required. Vendored sources, aliases resolving elsewhere, Plug'n'Play layouts and core-js-pure need separate matching and verification. Put the helper after plugins that set moduleSideEffects; a later plugin can override it.

Run the complete example

Download source, configuration and tests, extract the ZIP and enter its directory. With Node.js 18 or newer:

npm ci --ignore-scripts
npm test
npm run build
node -e "console.log(require('./dist/example.cjs').primitiveResults())"

The last command prints [ false, true, true, true ]. The example imports the four Object modules and executes the resulting bundle. Versions are pinned to Rollup 4.63.5, CommonJS plugin 29.0.3, node-resolve 16.0.3 and core-js 3.50.0.

Four scripted checks passed on Node.js 24.19.0 / macOS. They compare the default build with the configuration, observe the actual feature-detection calls before application operations, verify CommonJS conversion and module flags, repeat primitive and object operations, check a cached rebuild, and confirm unused application code is still removed. Removing the helper makes two of these checks fail.

The older-native case uses a separate Node VM with Object methods that reject primitive arguments. It demonstrates the detection and replacement path; it is not a run in a real older browser. Modern native behavior is checked separately. Windows, other core-js versions, other bundlers and downstream minifiers were not tested. Validate your final production bundle on your supported browsers, especially if another tool optimizes it afterward.

Keeping all included core-js statements increases bundle size. Global treeshake: false is a broader fallback, but also keeps unused application code. This example checks that distinction; it does not claim a particular size saving for your app. After upgrading Rollup, rerun your compatibility checks and reassess this temporary configuration.

The example and helper are available under the repository's MIT license.

Optional coffee

If this saves you some time and you'd like to buy me a coffee, thank you. It is entirely optional; the example, tests and guide are free.

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