cjs-module-lexer

What is cjs-module-lexer?

The cjs-module-lexer package is designed to analyze CommonJS modules to extract export and import information. It is particularly useful for tools that need to understand the structure of a module without executing it, such as bundlers or module loaders.

What are cjs-module-lexer's main functionalities?

Parse require statements

This feature allows you to parse the source code of a CommonJS module to identify all the require statements. It helps in understanding dependencies of the module.

const { parse } = require('cjs-module-lexer');
const source = "const x = require('some-module');";
const result = parse(source);
console.log(result);

Extract exports

This functionality enables the extraction of all export statements from a CommonJS module. It is useful for tools that need to generate a list of all exports from a module.

const { parse } = require('cjs-module-lexer');
const source = "exports.a = 1; module.exports.b = 2;";
const result = parse(source);
console.log(result);

Other packages similar to cjs-module-lexer

acorn

Acorn is a JavaScript parser that can parse ECMAScript code. It is similar to cjs-module-lexer in that it helps in understanding the structure of JavaScript code. However, Acorn supports ECMAScript syntax broadly, whereas cjs-module-lexer specifically targets CommonJS module syntax.

es-module-lexer

es-module-lexer is designed to parse ESM (ECMAScript Modules). It is similar to cjs-module-lexer but focuses on the newer ES module system rather than CommonJS. This makes es-module-lexer more suitable for projects that use native JavaScript modules.

README

CJS Module Lexer

A very fast JS CommonJS module syntax lexer used to detect the most likely list of named exports of a CommonJS module.

Outputs the list of named exports (exports.name = ...) and possible module reexports (module.exports = require('...')), including the common transpiler variations of these cases.

Forked from https://github.com/guybedford/es-module-lexer.

Comprehensively handles the JS language grammar while remaining small and fast. - ~90ms per MB of JS cold and ~15ms per MB of JS warm, see benchmarks for more info.

Usage

npm install cjs-module-lexer

For use in CommonJS:

const parse = require('cjs-module-lexer');

const { exports, reexports } = parse(`
  // named exports detection
  module.exports.a = 'a';
  (function () {
    exports.b = 'b';
  })();
  Object.defineProperty(exports, 'c', { value: 'c' });
  /* exports.d = 'not detected'; */

  // reexports detection
  if (maybe) module.exports = require('./dep1.js');
  if (another) module.exports = require('./dep2.js');

  // literal exports assignments
  module.exports = { a, b: c, d, 'e': f }

  // __esModule detection
  Object.defineProperty(module.exports, '__esModule', { value: true })
`);

// exports === ['a', 'b', 'c', '__esModule']
// reexports === ['./dep1.js', './dep2.js']

When using the ESM version, Wasm is supported instead:

import { parse, init } from 'cjs-module-lexer';
// init needs to be called and waited upon
await init();
const { exports, reexports } = parse(source);

The Wasm build is around 1.5x faster and without a cold start.

Grammar

CommonJS exports matches are run against the source token stream.

The token grammar is:

IDENTIFIER: As defined by ECMA-262, without support for identifier `\` escapes, filtered to remove strict reserved words:
            "implements", "interface", "let", "package", "private", "protected", "public", "static", "yield", "enum"

STRING_LITERAL: A `"` or `'` bounded ECMA-262 string literal.

IDENTIFIER_STRING: ( `"` IDENTIFIER `"` | `'` IDENTIFIER `'` )

COMMENT_SPACE: Any ECMA-262 whitespace, ECMA-262 block comment or ECMA-262 line comment

MODULE_EXPORTS: `module` COMMENT_SPACE `.` COMMENT_SPACE `exports`

EXPORTS_IDENTIFIER: MODULE_EXPORTS_IDENTIFIER | `exports`

EXPORTS_DOT_ASSIGN: EXPORTS_IDENTIFIER COMMENT_SPACE `.` COMMENT_SPACE IDENTIFIER COMMENT_SPACE `=`

EXPORTS_LITERAL_COMPUTED_ASSIGN: EXPORTS_IDENTIFIER COMMENT_SPACE `[` COMMENT_SPACE IDENTIFIER_STRING COMMENT_SPACE `]` COMMENT_SPACE `=`

EXPORTS_LITERAL_PROP: (IDENTIFIER (COMMENT_SPACE `:` COMMENT_SPACE IDENTIFIER)?) | (IDENTIFIER_STRING COMMENT_SPACE `:` COMMENT_SPACE IDENTIFIER)

EXPORTS_MEMBER: EXPORTS_DOT_ASSIGN | EXPORTS_LITERAL_COMPUTED_ASSIGN

EXPORTS_DEFINE: `Object` COMMENT_SPACE `.` COMMENT_SPACE `defineProperty COMMENT_SPACE `(` EXPORTS_IDENTIFIER COMMENT_SPACE `,` COMMENT_SPACE IDENTIFIER_STRING

EXPORTS_LITERAL: MODULE_EXPORTS COMMENT_SPACE `=` COMMENT_SPACE `{` COMMENT_SPACE (EXPORTS_LITERAL_PROP COMMENT_SPACE `,` COMMENT_SPACE)+ `}`

REQUIRE: `require` COMMENT_SPACE `(` COMMENT_SPACE STRING_LITERAL COMMENT_SPACE `)`

EXPORTS_ASSIGN: (`var` | `const` | `let`) IDENTIFIER `=` REQUIRE

MODULE_EXPORTS_ASSIGN: MODULE_EXPORTS COMMENT_SPACE `=` COMMENT_SPACE REQUIRE

EXPORT_STAR: (`__export` | `__exportStar`) `(` REQUIRE

EXPORT_STAR_LIB: `Object.keys(` IDENTIFIER$1 `).forEach(function (` IDENTIFIER$2 `) {`
  (
    `if (` IDENTIFIER$2 `===` ( `'default'` | `"default"` ) `||` IDENTIFIER$2 `===` ( '__esModule' | `"__esModule"` ) `) return` `;`? |
    `if (` IDENTIFIER$2 `!==` ( `'default'` | `"default"` ) `)`
  )
  (
    EXPORTS_IDENTIFIER `[` IDENTIFIER$2 `] =` IDENTIFIER$1 `[` IDENTIFIER$2 `]` `;`? |
    `Object.defineProperty(` EXPORTS_IDENTIFIER `, ` IDENTIFIER$2 `, { enumerable: true, get: function () { return ` IDENTIFIER$1 `[` IDENTIFIER$2 `]` `;`? } })` `;`?
  )
  `})`

• The returned export names are the matched IDENTIFIER and IDENTIFIER_STRING slots for all EXPORTS_MEMBER, EXPORTS_DEFINE and EXPORTS_LITERAL matches.
• The reexport specifiers are taken to be the STRING_LITERAL slots of all MODULE_EXPORTS_ASSIGN as well as all top-level EXPORT_STAR REQUIRE matches and EXPORTS_ASSIGN matches whose IDENTIFIER also matches the first IDENTIFIER in EXPORT_STAR_LIB.

Parsing Examples

Named Exports Parsing

The basic matching rules for named exports are exports.name, exports['name'] or Object.defineProperty(exports, 'name', ...). This matching is done without scope analysis and regardless of the expression position:

// DETECTS EXPORTS: a, b, c
(function (exports) {
  exports.a = 'a'; 
  exports['b'] = 'b';
  Object.defineProperty(exports, 'c', { value: 'c' });
})(exports);

Because there is no scope analysis, the above detection may overclassify:

// DETECTS EXPORTS: a, b, c
(function (exports, Object) {
  exports.a = 'a';
  exports['b'] = 'b';
  if (false)
    Object.defineProperty(exports, 'c', { value: 'c' });
})(NOT_EXPORTS, NOT_OBJECT);

It will in turn underclassify in cases where the identifiers are renamed:

// DETECTS: NO EXPORTS
(function (e, defineProperty) {
  e.a = 'a';
  e['b'] = 'b';
  defineProperty(e, 'c', { value: 'c' });
})(exports, defineProperty);

Exports Object Assignment

A best-effort is made to detect module.exports object assignments, but because this is not a full parser, arbitrary expressions are not handled in the object parsing process.

Simple object definitions are supported:

// DETECTS EXPORTS: a, b, c
module.exports = {
  a,
  b: 'c',
  c: c
};

Object properties that are not identifiers or string expressions will bail out of the object detection:

// DETECTS EXPORTS: a, b
module.exports = {
  a,
  b: require('c'),
  c: "not detected since require('c') above bails the object detection"
}

Object.defineProperties is not currently supported either.

module.exports reexport assignment

Any module.exports = require('mod') assignment is detected as a reexport:

// DETECTS REEXPORTS: a, b, c
module.exports = require('a');
(module => module.exports = require('b'))(NOT_MODULE);
if (false) module.exports = require('c');

As a result, the total list of exports would be inferred as the union of all of these reexported modules, which can lead to possible over-classification.

Transpiler Re-exports

For named exports, transpiler output works well with the rules described above.

But for star re-exports, special care is taken to support common patterns of transpiler outputs from Babel and TypeScript as well as bundlers like RollupJS. These reexport and star reexport patterns are restricted to only be detected at the top-level as provided by the direct output of these tools.

For example, export * from 'external' is output by Babel as:

"use strict";

exports.__esModule = true;

var _external = require("external");

Object.keys(_external).forEach(function (key) {
  if (key === "default" || key === "__esModule") return;
  exports[key] = _external[key];
});

Where the var _external = require("external") is specifically detected as well as the Object.keys(_external) statement, down to the exact for of that entire expression including minor variations of the output. The _external and key identifiers are carefully matched in this detection.

Similarly for TypeScript, export * from 'external' is output as:

"use strict";
function __export(m) {
    for (var p in m) if (!exports.hasOwnProperty(p)) exports[p] = m[p];
}
Object.defineProperty(exports, "__esModule", { value: true });
__export(require("external"));

Where the __export(require("external")) statement is explicitly detected as a reexport, including variations tslib.__export and __exportStar.

Environment Support

Node.js 10+, and all browsers with Web Assembly support.

JS Grammar Support

• Token state parses all line comments, block comments, strings, template strings, blocks, parens and punctuators.
• Division operator / regex token ambiguity is handled via backtracking checks against punctuator prefixes, including closing brace or paren backtracking.
• Always correctly parses valid JS source, but may parse invalid JS source without errors.

Benchmarks

Benchmarks can be run with npm run bench.

Current results:

JS Build:

Module load time
> 2ms
Cold Run, All Samples
test/samples/*.js (3635 KiB)
> 333ms

Warm Runs (average of 25 runs)
test/samples/angular.js (1410 KiB)
> 16.48ms
test/samples/angular.min.js (303 KiB)
> 5.36ms
test/samples/d3.js (553 KiB)
> 8.32ms
test/samples/d3.min.js (250 KiB)
> 4.28ms
test/samples/magic-string.js (34 KiB)
> 1ms
test/samples/magic-string.min.js (20 KiB)
> 0.36ms
test/samples/rollup.js (698 KiB)
> 10.48ms
test/samples/rollup.min.js (367 KiB)
> 6.64ms

Warm Runs, All Samples (average of 25 runs)
test/samples/*.js (3635 KiB)
> 49.28ms

Wasm Build:

Module load time
> 11ms
Cold Run, All Samples
test/samples/*.js (3635 KiB)
> 48ms

Warm Runs (average of 25 runs)
test/samples/angular.js (1410 KiB)
> 12.32ms
test/samples/angular.min.js (303 KiB)
> 3.76ms
test/samples/d3.js (553 KiB)
> 6.08ms
test/samples/d3.min.js (250 KiB)
> 3ms
test/samples/magic-string.js (34 KiB)
> 0.24ms
test/samples/magic-string.min.js (20 KiB)
> 0ms
test/samples/rollup.js (698 KiB)
> 7.2ms
test/samples/rollup.min.js (367 KiB)
> 4.2ms

Warm Runs, All Samples (average of 25 runs)
test/samples/*.js (3635 KiB)
> 33.6ms

Wasm Build Steps

To build download the WASI SDK from https://github.com/CraneStation/wasi-sdk/releases.

The Makefile assumes the existence of "wasi-sdk-10.0", "binaryen" and "wabt" (both optional) as sibling folders to this project.

The build through the Makefile is then run via make lib/lexer.wasm, which can also be triggered via npm run build-wasm to create dist/lexer.js.

On Windows it may be preferable to use the Linux subsystem.

After the Web Assembly build, the CJS build can be triggered via npm run build.

Optimization passes are run with Binaryen prior to publish to reduce the Web Assembly footprint.

License

MIT