Missing core‑js polyfills after @babel/preset‑env compilation

No, @babel/preset-env does not automatically emit polyfills if the useBuiltIns option is omitted. By default, it only transforms modern JavaScript syntax (e.g., arrow functions to standard functions) but does not provide missing global objects or methods (like Promise) required by older runtimes.

Polyfill Injection Strategies

To resolve the ReferenceError: Promise is not defined, you must explicitly configure useBuiltIns. The behavior differs significantly between the two primary options:

Option	Mechanism	Requirement
'entry'	Replaces a single global import with a list of all polyfills needed for the target environment.	You must add import "core-js/stable"; at the very top of your entry file.
'usage'	Analyzes each file and injects import statements only for the specific features used in that file.	No manual imports required; Babel handles injection automatically.

Configuration File Precedence

When both babel.config.js and .babelrc exist, Babel 7 treats them differently based on scope:

• babel.config.js: A project-wide configuration file. It is typically used for root-level settings and is applied to all files, including those in node_modules if configured to do so.
• .babelrc: A file-relative configuration. It applies only to the directory it resides in and its subdirectories.

If both are present, Babel merges them. However, settings in .babelrc generally override those in babel.config.js for files within that specific directory. If your preset-env options are split across both files, the resulting configuration may be inconsistent.

Resolution Steps

1. Consolidate Config: Move all preset-env options into a single babel.config.js to avoid override conflicts.
2. Enable Polyfills: Update your configuration to include useBuiltIns:

   {
     "presets": [
       [
         "@babel/preset-env",
         {
           "targets": { "node": "10" },
           "useBuiltIns": "usage",
           "corejs": 3
         }
       ]
     ]
   }

3. Verify Dependencies: Ensure core-js is listed in your package.json dependencies (not devDependencies), as it must be present in the production runtime.

Verification

To confirm which configuration is actually being applied, run the following command in your terminal:

npx babel --show-config

Check the output to ensure useBuiltIns is correctly set to usage or entry.

Diagnostic Detail Needed: Are you compiling files inside node_modules? If so, .babelrc files within those packages may be overriding your project-wide settings.