What is the default polyfill behavior of @babel/preset-env when target environments are unspecified?

Answer to the Core Question

When you run @babel/preset-env without a targets field and useBuiltIns is **not** set, Babel does not add any polyfills. The preset only transpiles syntax features; it leaves runtime features untouched.

What Happens If useBuiltIns Is Set?

• useBuiltIns: 'usage' or 'entry' forces Babel to inject core‑js polyfills.
• Without an explicit targets, preset‑env falls back to the built‑in Browserslist defaults: the last two releases of each browser, browsers with >1% market share, and browsers that are not dead.
• Only the polyfills required for the features actually used in your code are included, based on that default target set.

How to Verify What Babel Is Doing

1. Check your Babel config (e.g., .babelrc or babel.config.js) for a useBuiltIns key.

2. Run Babel with the --verbose flag:

   npx babel src/index.js --verbose
   

   The output lists any core‑js modules that were injected.

3. To see the target set that preset‑env is using, run:

   npx browserslist --verbose
   

   When no .browserslistrc or browserslist field in package.json is present, the list should match the default query mentioned above.

4. Inspect the transpiled code for imports such as import 'core-js/modules/es.array.flat.js'; to confirm polyfills were added.

Missing Diagnostic Detail

Does your configuration set useBuiltIns to 'usage' or 'entry'?

If not, no polyfills will be injected regardless of the code.