cjs-module-lexer

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 `'` )

MODULE_EXPORTS: `module` `.` `exports`

EXPORTS_IDENTIFIER: MODULE_EXPORTS_IDENTIFIER | `exports`

EXPORTS_DOT_ASSIGN: EXPORTS_IDENTIFIER `.` IDENTIFIER `=`

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

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

EXPORTS_SPREAD: `...` (IDENTIFIER | REQUIRE)

EXPORTS_MEMBER: EXPORTS_DOT_ASSIGN | EXPORTS_LITERAL_COMPUTED_ASSIGN

EXPORTS_DEFINE: `Object` `.` `defineProperty `(` IDENTIFIER_STRING `, {`
  (`enumerable: true,`)?
  (
    `value:` |
    `get:` `function`? `()` {` return IDENTIFIER (`.` IDENTIFIER | `[` IDENTIFIER_STRING `]`)? `;`? `}`
  )
  `})`

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

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

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

MODULE_EXPORTS_ASSIGN: MODULE_EXPORTS `=` 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"` ) `)`
  )
  (
    `if (` IDENTIFIER$2 `in` EXPORTS_IDENTIFIER `&&` EXPORTS_IDENTIFIER `[` IDENTIFIER$2 `] ===` IDENTIFIER$1 `[` IDENTIFIER$2 `]) return` `;`?
  )?
  (
    EXPORTS_IDENTIFIER `[` IDENTIFIER$2 `] =` IDENTIFIER$1 `[` IDENTIFIER$2 `]` `;`? |
    `Object.defineProperty(` EXPORTS_IDENTIFIER `, ` IDENTIFIER$2 `, { enumerable: true, get: function () { return ` IDENTIFIER$1 `[` IDENTIFIER$2 `]` `;`? } })` `;`?
  )
  `})`

Spacing between tokens is taken to be any ECMA-262 whitespace, ECMA-262 block comment or ECMA-262 line comment.

• The returned export names are taken to be the combination of the IDENTIFIER and IDENTIFIER_STRING slots for all EXPORTS_MEMBER, EXPORTS_LITERAL and EXPORTS_DEFINE matches.
• The reexport specifiers are taken to be the the combination of:
  1. The REQUIRE matches of the last matched of either MODULE_EXPORTS_ASSIGN or EXPORTS_LITERAL.
  2. 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
(function (exports) {
  exports.a = 'a'; 
  exports['b'] = 'b';
})(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)
    exports.c = 'c';
})(NOT_EXPORTS, NOT_OBJECT);

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

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

Object.defineProperty is detected for specifically value and getter forms returning an identifier or member expression:

// DETECTS: a, b, c, d, __esModule
Object.defineProperty(exports, 'a', {
  enumerable: true,
  get: function () {
    return q.p;
  }
});
Object.defineProperty(exports, 'b', {
  enumerable: true,
  get: function () {
    return q['p'];
  }
});
Object.defineProperty(exports, 'c', {
  enumerable: true,
  get () {
    return b;
  }
});
Object.defineProperty(exports, 'd', { value: 'd' });
Object.defineProperty(exports, '__esModule', { value: true });

Alternative object definition structures or getter function bodies are not detected:

// DETECTS: NO EXPORTS
Object.defineProperty(exports, 'a', {
  enumerable: false,
  get () {
    return p;
  }
});
Object.defineProperty(exports, 'b', {
  configurable: true,
  get () {
    return p;
  }
});
Object.defineProperty(exports, 'c', {
  get: () => p
});
Object.defineProperty(exports, 'd', {
  enumerable: true,
  get: function () {
    return dynamic();
  }
});
Object.defineProperty(exports, 'e', {
  enumerable: true,
  get () {
    return 'str';
  }
});

Object.defineProperties is also not supported.

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': b,
  c: c,
  ...d
};

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

// DETECTS EXPORTS: a, b
module.exports = {
  a,
  ...d,
  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, but only the last one is returned:

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

This is to avoid over-classification in Webpack bundles with externals which include module.exports = require('external') in their source for every external dependency.

In exports object assignment, any spread of require() are detected as multiple separate reexports:

// DETECTS REEXPORTS: a, b
module.exports = require('ignored');
module.exports = {
  ...require('a'),
  ...require('b')
};

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
> 5ms
Cold Run, All Samples
test/samples/*.js (3635 KiB)
> 323ms

Warm Runs (average of 25 runs)
test/samples/angular.js (1410 KiB)
> 14.84ms
test/samples/angular.min.js (303 KiB)
> 4.8ms
test/samples/d3.js (553 KiB)
> 7.84ms
test/samples/d3.min.js (250 KiB)
> 4ms
test/samples/magic-string.js (34 KiB)
> 0.72ms
test/samples/magic-string.min.js (20 KiB)
> 0.4ms
test/samples/rollup.js (698 KiB)
> 9.32ms
test/samples/rollup.min.js (367 KiB)
> 6.52ms

Warm Runs, All Samples (average of 25 runs)
test/samples/*.js (3635 KiB)
> 44ms

Wasm Build:

Module load time
> 11ms
Cold Run, All Samples
test/samples/*.js (3635 KiB)
> 42ms

Warm Runs (average of 25 runs)
test/samples/angular.js (1410 KiB)
> 9.92ms
test/samples/angular.min.js (303 KiB)
> 3.2ms
test/samples/d3.js (553 KiB)
> 5.2ms
test/samples/d3.min.js (250 KiB)
> 2.52ms
test/samples/magic-string.js (34 KiB)
> 0.16ms
test/samples/magic-string.min.js (20 KiB)
> 0.04ms
test/samples/rollup.js (698 KiB)
> 6.44ms
test/samples/rollup.min.js (367 KiB)
> 3.96ms

Warm Runs, All Samples (average of 25 runs)
test/samples/*.js (3635 KiB)
> 30.48ms

Wasm Build Steps

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

The Makefile assumes the existence of "wasi-sdk-11.0" and "wabt" (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