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 = ...), 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. - ~10ms per MB of JS cold and ~5ms per MB of JS warm, see benchmarks for more info.

Usage

npm install cjs-module-lexer

For use in CommonJS:

const { init, parse } = require('cjs-module-lexer');

(async () => {
  // either await init, or call parse asynchronously
  // this is necessary for the Web Assembly boot
  await init;

  const { exports, reexports, esModule } = 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');

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

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

An ES module version is also available from dist/lexer.js, automatically enabled via "exports":

Supported

1. All exports.a or module.exports.a usages
2. All Object.defineProperty(module.exports, 'name' or Object.defineProperty(exports, 'name' assignments
3. All module.exports = require('string') assignments

Not Supported

1. No scope analysis:

// "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);

1. No object parsing:

// These WONT be detected as exports
Object.defineProperties(exports, {
  a: { value: 'a' },
  b: { value: 'b' }
});

// These WONT be detected as exports
module.exports = {
  c: 'c',
  d: 'd'
}

Environment Support

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

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.

Limitations

The lexing approach is designed to deal with the full language grammar including RegEx / division operator ambiguity through backtracking and paren / brace tracking.

The only limitation to the reduced parser is that the "exports" list may not correctly gather all export identifiers in the following edge cases:

// Only "a" is detected as an export, "q" isn't
export var a = 'asdf', q = z;

// "b" is not detected as an export
export var { a: b } = asdf;

The above cases are handled gracefully in that the lexer will keep going fine, it will just not properly detect the export names above.

Benchmarks

Benchmarks can be run with npm run bench.

Current results:

Cold Run, All Samples
test/samples/*.js (3057 KiB)
> 24ms

Warm Runs (average of 25 runs)
test/samples/angular.js (719 KiB)
> 5.12ms
test/samples/angular.min.js (188 KiB)
> 3.04ms
test/samples/d3.js (491 KiB)
> 4.08ms
test/samples/d3.min.js (274 KiB)
> 2.04ms
test/samples/magic-string.js (34 KiB)
> 0ms
test/samples/magic-string.min.js (20 KiB)
> 0ms
test/samples/rollup.js (902 KiB)
> 5.92ms
test/samples/rollup.min.js (429 KiB)
> 3.08ms

Warm Runs, All Samples (average of 25 runs)
test/samples/*.js (3057 KiB)
> 17.4ms

Building

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

The Makefile assumes that the clang in PATH corresponds to LLVM 8 (provided by WASI SDK as well, or a standard clang 8 install can be used as well), and that ../wasi-sdk-6 contains the SDK as extracted above, which is important to locate the WASI sysroot.

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