Migrating a shared component library from core-js 2 to 3: global build or core-js-pure?

Answer

For a library that is consumed by applications which may independently load the global core-js build, the officially recommended approach is to use core-js-pure.

Confirmed facts

• The core-js documentation states that library authors should prefer core-js-pure because it does not mutate built‑in prototypes and lets the consumer decide whether a global polyfill is needed.
• When a library uses core-js-pure and an application also loads the global core-js build, the two can coexist safely: the pure imports provide the same functionality without touching globals, while the global build patches prototypes for the app.
• The stable entry‑point prefix for published packages is /stable/; it re‑exports only features that have reached stage 4 (finished proposals) and is the baseline recommended for libraries that must not include experimental proposals.
• Babel’s useBuiltIns: 'usage' with corejs: 3 works correctly when a dependency imports core-js-pure directly; Babel will add imports only for the actually used methods and will not duplicate them if the same methods are also provided by the global core-js loaded elsewhere.

Likely explanation

If both flavors end up in the same bundle you may see duplicate polyfill code, which increases bundle size but does not cause functional conflicts because core-js-pure does not modify globals and the global build’s patches are idempotent.

Steps for this migration

1. Replace all core-js/fn/... imports in the library with imports from core-js-pure/stable/ (e.g., import 'core-js-pure/stable/array/from').
2. Ensure the library’s package.json lists core-js-pure as a dependency (not the global core-js).
3. Consuming applications can continue to load the global core-js build if they need prototype patches; no additional configuration is required.
4. After building, inspect the bundle (e.g., with webpack‑bundle‑analyzer) to verify that no unnecessary duplicate core-js modules appear.

If you observe unexpected duplicate code, provide the bundle‑analysis report showing which core-js modules are duplicated; that detail would affect the recommendation to deduplicate imports.