Registry / serialization / cjs-module-lexer

cjs-module-lexer

library2.2.0jsnpmunverified

A fast CommonJS module syntax lexer that detects named exports and reexports from CJS modules, used in Node.js core for ESM interop. Current stable version: 2.2.0. Maintained by Node.js team; release cadence is irregular but active for parser bugs and performance improvements. Key differentiators: extremely fast (~90ms/MB cold, ~15ms/MB warm), frozen detection patterns to ensure backwards compatibility across Node.js versions, and support for transpiler variations (e.g., TypeScript, Babel). Provides both Node.js (CJS) and Wasm (ESM) builds, with TypeScript declarations included.

serialization

npm install cjs-module-lexer

github ↗package ↗

INSTALL

IMPORT

SIG · CJS-MODULE-LEXER

cjs-module-lexer

serializationjavascriptv2.2.0

harness data pending

Install & Compatibility

Where this runs

No compatibility data collected yet for this library.

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

parse

✓ import { parse, init } from 'cjs-module-lexer'

✗ const parse = require('cjs-module-lexer').parse

For ESM, init() must be awaited before first parse() call. For CJS, init is automatically handled.

init

✓ import { init } from 'cjs-module-lexer'

✗ const { init } = require('cjs-module-lexer')

init() is only needed for Wasm build (ESM). In CJS, it's a no-op promise.

parse

✓ const { parse } = require('cjs-module-lexer'); parse(source)

✗ const cjsModuleLexer = require('cjs-module-lexer'); cjsModuleLexer.parse(source)

CJS import: destructure parse from the module. No need to call init().

Demonstrates ESM usage with Wasm: import parse and init, await init(), parse a CJS source, and obtain exports/reexports arrays.

import { parse, init } from 'cjs-module-lexer'; const source = ` module.exports.a = 'a'; if (maybe) module.exports = require('./dep1.js'); module.exports = { b, c: d }; `; await init(); const { exports, reexports } = parse(source); console.log(exports); // ['a', 'b', 'c', '__esModule'] console.log(reexports); // ['./dep1.js']

Debug

Known issues

breakingIn v2.0.0, the Wasm build was introduced and the ESM API changed: init() must be called before parse(). Previously, init() was not required and Wasm was not used.

fix

Add await init() before calling parse() in ESM environments. CJS users are unaffected.

affects: >=2.0.0

deprecatedThe detection patterns are frozen; new export patterns will not be added. This ensures backwards compatibility but means some modern CJS patterns may not be detected.

fix

Do not rely on detection of non-standard patterns. Ensure your code uses the documented export forms (e.g., exports.X = ..., module.exports = { ... }, Object.defineProperty(exports, ...)).

affects: >=1.0.0

gotchaThe lexer does not handle all JavaScript edge cases (e.g., dynamic require, conditional exports inside functions). It uses a token grammar that may miss exports with unusual whitespace or comments.

fix

Test your modules with the lexer to ensure expected exports are detected. For complex cases, consider alternative tools.

affects: >=1.0.0

gotchaThe `__esModule` export is always included in the exports array if Object.defineProperty(module.exports, '__esModule', ...) is present. This may be unexpected for some users.

fix

Filter out '__esModule' from exports if not needed: exports.filter(e => e !== '__esModule')

affects: >=1.0.0

breakingVersion 1.4.2 included a change that moved to a common wasm-builder, but this was later refined. Users upgrading from 1.x should test exports detection thoroughly.

fix

Upgrade to 2.2.0 or later, and verify export detection.

affects: 1.4.2

Errors

Common errors & fixes

Error: Must call init() before parse()

Using ESM import with Wasm build but not calling await init() first.

fix

Add await init(); before any parse() call in ESM context.

TypeError: parse is not a function

Importing cjs-module-lexer incorrectly in CJS, e.g., using default import or wrong destructure.

fix

Use const { parse } = require('cjs-module-lexer'); in CommonJS.

ReferenceError: exports is not defined

Trying to access exports in a context where the CJS wrapper (function(exports, module, require) { ... }) is not provided.

fix

Ensure source code is wrapped in a CommonJS module wrapper when lexing; the lexer expects valid CJS syntax.

Warning: cjs-module-lexer detected a reexport but the path './foo' is not quoted?

Using module.exports = require('./foo') without quotes around the path? Actually, the lexer expects string literals.

fix

Use require('./foo') with a string literal; ensure no dynamic expressions.

Upgrade

Version history

2.2.0latest on npm

Audit

Dependencies

No dependency data recorded yet.

Agent activity

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Resources

githubhttps://github.com/nodejs/cjs-module-lexer ↗

packagehttps://www.npmjs.com/package/cjs-module-lexer ↗