How Node.js Distinguishes ES Modules from CommonJS After Dropping --experimental-modules

Node.js 13.2.0, released on the 21st of this month, removed the --experimental-modules flag, indicating that ES module support has moved to a more stable stage.

Node.js distinguishes ES modules from CommonJS modules in the following cases:

File extension is .mjs.

The type field in package.json is set to "module".

The runtime is started with --input-type=module.

In all other situations the file is treated as a CommonJS module.

Conversely, a file is forced to be recognized as CommonJS when any of these conditions are met:

File extension is .cjs.

The type field in package.json is set to "commonjs".

The runtime is started with --input-type=commonjs.

Example:

// package.json contains "type": "module"
import './test/aaa.js'; // aaa.js is loaded as an ES module because the current package.json has type=module
import 'test-aaa'; // loading depends on test-aaa's own package.json type field
import './node_modules/test-aaa/index.js'; // same logic applies

The official documentation currently marks this feature with “Stability: 1”, meaning it is still experimental, but the removal of the flag suggests it will soon reach “Stability: 2” and become fully stable.

For more details, refer to the original Node.js 13.2.0 ESM documentation.