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.
This project is used in Node.js core for detecting the named exports available when importing a CJS module into ESM, and is maintained for this purpose.
PRs will be accepted and upstreamed for parser bugs, performance improvements or new syntax support only.
Detection patterns for this project are frozen. This is because adding any new export detection patterns would result in fragmented backwards-compatibility. Specifically, it would be very difficult to figure out why an ES module named export for CommonJS might work in newer Node.js versions but not older versions. This problem would only be discovered downstream of module authors, with the fix for module authors being to then have to understand which patterns in this project provide full backwards-compatibily. Rather, by fully freezing the detected patterns, if it works in any Node.js version it will work in any other. Build tools can also reliably treat the supported syntax for this project as a part of their output target for ensuring syntax support.
npm install cjs-module-lexer
For use in CommonJS:
```js const { parse } = require('cjs-module-lexer');
// init return a promise for parity with the ESM API, but you do not have to call it
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:
js
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.
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.
MODULE_EXPORTS: module . exports
EXPORTS_IDENTIFIER: MODULE_EXPORTS_IDENTIFIER | exports
EXPORTS_DOT_ASSIGN: EXPORTS_IDENTIFIER . IDENTIFIER =
EXPORTS_LITERAL_COMPUTED_ASSIGN: EXPORTS_IDENTIFIER [ STRING_LITERAL ] =
EXPORTS_LITERAL_PROP: (IDENTIFIER (: IDENTIFIER)?) | (STRING_LITERAL : IDENTIFIER)
EXPORTS_SPREAD: ... (IDENTIFIER | REQUIRE)
EXPORTS_MEMBER: EXPORTS_DOT_ASSIGN | EXPORTS_LITERAL_COMPUTED_ASSIGN
EXPORTS_DEFINE: Object . defineProperty(EXPORTS_IDENFITIER,` STRING_LITERAL
EXPORTS_DEFINE_VALUE: EXPORTS_DEFINE , {
(enumerable: true,)?
(
value: |
get (: function IDENTIFIER? )? () { return IDENTIFIER (. IDENTIFIER | [ STRING_LITERAL ])? ;? } ,?
)
})
EXPORTS_LITERAL: MODULE_EXPORTS = { (EXPORTS_LITERAL_PROP | EXPORTS_SPREAD) ,)+ }
REQUIRE: require ( STRING_LITERAL )
EXPORTS_ASSIGN: (var | const | let) IDENTIFIER = (_interopRequireWildcard ()? 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 (Object .prototype? .hasOwnProperty.call( IDENTIFIER , IDENTIFIER$2 )) return ;?)?
(if ( IDENTIFIER$2 in EXPORTS_IDENTIFIER && EXPORTS_IDENTIFIER [ IDENTIFIER$2 ] === IDENTIFIER$1 [ IDENTIFIER$2 ]) return ;)?
)?
) |
if ( IDENTIFIER$2 !== ( 'default' | "default" ) (&& ! (Object .prototype? .hasOwnProperty.call( IDENTIFIER , IDENTIFIER$2 ) | IDENTIFIER .hasOwnProperty( IDENTIFIER$2 )))? )
)
(
EXPORTS_IDENTIFIER [ IDENTIFIER$2 ] = IDENTIFIER$1 [ IDENTIFIER$2 ] ;? |
Object.defineProperty( EXPORTS_IDENTIFIER , IDENTIFIER$2 , { enumerable: true, get (: function IDENTIFIER? )? () { 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.
IDENTIFIER and STRING_LITERAL slots for EXPORTS_MEMBER and EXPORTS_LITERAL matches.STRING_LITERAL slot for all EXPORTS_DEFINE_VALUE matches where that same string is not an EXPORTS_DEFINE match that is not also an EXPORTS_DEFINE_VALUE match.REQUIRE matches of the last matched of either MODULE_EXPORTS_ASSIGN or EXPORTS_LITERAL.EXPORT_STAR REQUIRE matches and EXPORTS_ASSIGN matches whose IDENTIFIER also matches the first IDENTIFIER in EXPORT_STAR_LIB.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:
js
// DETECTS EXPORTS: a, b
(function (exports) {
exports.a = 'a';
exports['b'] = 'b';
})(exports);
Because there is no scope analysis, the above detection may overclassify:
js
// 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:
js
// 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:
js
// 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 });
Value properties are also detected specifically:
js
Object.defineProperty(exports, 'a', {
value: 'no problem'
});
To avoid matching getters that have side effects, any getter for an export name that does not support the forms above will opt-out of the getter matching:
```js // DETECTS: NO EXPORTS Object.defineProperty(exports, 'a', { get () { return 'nope'; } });
if (false) { Object.defineProperty(module.exports, 'a', { get () { return dynamic(); } }) } ```
Alternative object definition structures or getter function bodies are not detected:
js
// 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.
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:
js
// 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:
js
// 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.
Any module.exports = require('mod') assignment is detected as a reexport, but only the last one is returned:
js
// 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:
js
// DETECTS REEXPORTS: a, b
module.exports = require('ignored');
module.exports = {
...require('a'),
...require('b')
};
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:
```js "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:
js
"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.
Node.js 10+, and all browsers with Web Assembly support.
Benchmarks can be run with npm run bench.
Current results:
JS Build:
``` Module load time
4ms Cold Run, All Samples test/samples/*.js (3635 KiB) 299ms
Warm Runs (average of 25 runs) test/samples/angular.js (1410 KiB)
13.96ms test/samples/angular.min.js (303 KiB) 4.72ms test/samples/d3.js (553 KiB) 6.76ms test/samples/d3.min.js (250 KiB) 4ms test/samples/magic-string.js (34 KiB) 0.64ms test/samples/magic-string.min.js (20 KiB) 0ms test/samples/rollup.js (698 KiB) 8.48ms test/samples/rollup.min.js (367 KiB) 5.36ms
Warm Runs, All Samples (average of 25 runs) test/samples/*.js (3635 KiB)
40.28ms ```
Wasm Build: ``` Module load time
10ms Cold Run, All Samples test/samples/*.js (3635 KiB) 43ms
Warm Runs (average of 25 runs) test/samples/angular.js (1410 KiB)
9.32ms test/samples/angular.min.js (303 KiB) 3.16ms test/samples/d3.js (553 KiB) 5ms test/samples/d3.min.js (250 KiB) 2.32ms test/samples/magic-string.js (34 KiB) 0.16ms test/samples/magic-string.min.js (20 KiB) 0ms test/samples/rollup.js (698 KiB) 6.28ms test/samples/rollup.min.js (367 KiB) 3.6ms
Warm Runs, All Samples (average of 25 runs) test/samples/*.js (3635 KiB)
27.76ms ```
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.
MIT