CJS/ESM interop

How Node bridges CommonJS and ES modules.

CJS preparse and synthetic namespaces

When ESM imports CJS, Node synthesizes an ESM namespace from static CJS source analysis before evaluating the CJS module.

In v24.15.0, the CJS translator calls cjsPreparseModuleExports before building the wrapper (translators.js#L212).
The parser returns exports and reexports, following reexports recursively (translators.js#L393-L418).
Node marks CJS modules cached by the ESM loader with kIsCachedByESMLoader (translators.js#L368); the CJS loader uses that marker during circular loads (loader.js#L1297-L1308).
Node v23 adds a synthetic 'module.exports' namespace key (v23.0.0 translators.js#L187).

The lexer implementation changed without changing the supported grammar:

v24.11.1 uses vendored cjs-module-lexer (grammar).
v24.14.0 uses internalBinding('cjs_lexer') through nodejs/node#61456, backed by Merve (grammar).

Node behavior	PR	Verified releases
The ESM `load` hook can return source for `format === 'commonjs'`.	nodejs/node#50825	v20.11.0, v21.3.0
CJS source returned by a load hook can be preparsed into a namespace.	nodejs/node#50825, #54769	`[20.11.0, 21.0.0)` and `>=21.3.0`

The load hook receives import-attributes context and reads CJS source at the boundary (v20.11.0 load.js#L113-L145). The translator then preparses CJS before namespace creation and evaluates via CJSModule._load (v20.11.0 translators.js#L190-L203).

Node behavior	PR	Verified releases or window
CJS `require()` can load eligible ESM instead of throwing `ERR_REQUIRE_ESM`.	nodejs/node#55085	v20.19.0, v22.12.0, v23.0.0
Normal `require(esm)` stops printing the experimental warning.	nodejs/node#56194	v20.19.0, v22.13.0, v23.5.0
Extensionless `.mjs` lookup was broken.	nodejs/node#55085, #55590	`[20.19.0, 20.19.5)`, `[22.12.0, 22.14.0)`
Synthetic CJS namespaces expose `'module.exports'`.	nodejs/node#53848	v23.0.0