cjs-module-lexer
Advanced tools
Package description
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.
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);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 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
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 = ...), whether the __esModule interop flag is used, and possible module reexports (module.exports = require('...')).
For an example of the performance, Angular 1 (720KiB) is fully parsed in 5ms, in comparison to the fastest JS parser, Acorn which takes over 100ms.
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.
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.
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 `]` `;`? } })` `;`?
)
`})`
IDENTIFIER and IDENTIFIER_STRING slots for all EXPORTS_MEMBER, EXPORTS_DEFINE and EXPORTS_LITERAL matches.STRING_LITERAL slots of all top-level MODULE_EXPORTS_ASSIGN and EXPORT_STAR REQUIRE matches as well as all EXPORTS_ASSIGN matches whose IDENTIFIER also matches the first IDENTIFIER in EXPORT_STAR_LIB// "a" WILL be detected as an export
(function (exports) {
exports.a = 'a';
})(notExports);
// "b" WONT be detected as an export
(function (m) {
m.a = 'a';
})(exports);
module.exports require assignment only handled at the base-level// OK
module.exports = require('./a.js');
// OK
if (condition)
module.exports = require('./b.js');
// NOT OK -> nested top-level detections not implemented
if (condition) {
module.exports = require('./c.js');
}
(function () {
module.exports = require('./d.js');
})();
// These WONT be detected as exports
Object.defineProperties(exports, {
a: { value: 'a' },
b: { value: 'b' }
});
module.exports = {
// These WILL be detected as exports
a: a,
b: b,
// This WILL be detected as an export
e: require('d'),
// These WONT be detected as exports
// because the object parser stops on the non-identifier
// expression "require('d')"
f: 'f'
}
// './x' detected as star export
var x = require('./x');
Object.keys(x).forEach(function (k) {
if (k !== 'default') Object.defineProperty(exports, k, {
enumerable: true,
get: function () {
return x[k];
}
});
});
// './y' detected as star export
let y = require('./y');
Object.keys(y).forEach(function (kk) {
if (kk !== 'default') exports[kk] = y[kk];
});
// './z' NOT detected as star export
let z = require('./z');
for (const key of Object.keys(x)) {
exports[key] = x[key];
}
These patterns can be updated over time to match modern transpiler outputs.
Node.js 10+, and all browsers with Web Assembly support.
Benchmarks can be run with npm run bench.
Current results:
Module load time
> 2ms
Cold Run, All Samples
test/samples/*.js (3635 KiB)
> 318ms
Warm Runs (average of 25 runs)
test/samples/angular.js (1410 KiB)
> 18.64ms
test/samples/angular.min.js (303 KiB)
> 5.96ms
test/samples/d3.js (553 KiB)
> 8.88ms
test/samples/d3.min.js (250 KiB)
> 4.88ms
test/samples/magic-string.js (34 KiB)
> 1ms
test/samples/magic-string.min.js (20 KiB)
> 0.32ms
test/samples/rollup.js (698 KiB)
> 11.68ms
test/samples/rollup.min.js (367 KiB)
> 7.84ms
Warm Runs, All Samples (average of 25 runs)
test/samples/*.js (3635 KiB)
> 54.48ms
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.
MIT
FAQs
Lexes CommonJS modules, returning their named exports metadata
We found that cjs-module-lexer demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 2 open source maintainers collaborating on the project.
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