In Nuxt 3, there's no main.ts entry point like in plain Vue 3 apps—instead, you can safely import core-js in a dedicated Nuxt plugin (executed before app initialization) to ensure polyfills load early and consistently across all browser environments.

in nuxt 3, there's no `main.ts` entry point like in plain vue 3 apps—instead, you can safely import `core-js` in a dedicated nuxt plugin (executed before app initialization) to ensure polyfills load early and consistently across all browser environments.

Nuxt 3 uses a plugin-based boot system where files in the plugins/ directory are automatically registered and executed during app startup. To polyfill older browsers reliably with core-js, you should create a client-side plugin—because polyfills are only needed in the browser, not during SSR.

✅ Step-by-step setup:

1. Install core-js:

   npm install core-js
   # or
   yarn add core-js

2. Create a new plugin file: plugins/polyfill.client.ts

   // plugins/polyfill.client.ts
   import 'core-js/stable'
   import 'regenerator-runtime/runtime'

? Note the .client.ts suffix — this tells Nuxt to run this plugin only on the client, avoiding SSR errors and unnecessary bundle bloat on the server.

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3. Ensure the plugin is auto-registered — no extra configuration needed, as long as it’s placed in plugins/ and follows the naming convention (*.client.ts or *.server.ts).

? Why not app.vue?
Importing polyfills directly in app.vue (e.g., in <script setup> or onBeforeMount) is too late: many native APIs may already be used by Nuxt’s internal runtime or composables before component mount. Early, global polyfilling is essential for correctness.

⚠️ Important considerations:

• Avoid importing core-js in nuxt.config.ts, composables/, or shared utilities — those may execute during SSR or build time, leading to errors or duplicated polyfills.
• Prefer granular imports (e.g., core-js/stable/array/find, core-js/stable/promise) over the full core-js/stable if bundle size matters — but verify coverage for your target browsers (e.g., IE11, Safari < 14).
• Always test in your oldest supported browser — especially for features like Array.from, Promise.finally, or optional chaining fallbacks.

✅ Bonus: For fine-grained control (e.g., conditional polyfilling), you can wrap logic in a dynamic check:

// plugins/polyfill.client.ts
if (typeof Promise === 'undefined' || !Array.prototype.find) {
  import('core-js/stable')
  import('regenerator-runtime/runtime')
}

But note: dynamic import() delays execution — use only if strictly necessary; static imports are safer and more reliable for foundational polyfills.

In summary: Use plugins/polyfill.client.ts with static core-js/stable import — it’s the most robust, Nuxt-native, and SSR-safe approach.