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

Low-overhead lexer dedicated to ES module parsing for fast analysis
https://github.com/guybedford/es-module-lexer

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Low-overhead lexer dedicated to ES module parsing for fast analysis

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# ES Module Lexer



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A JS module syntax lexer used in [es-module-shims](https://github.com/guybedford/es-module-shims).



Outputs the list of exports and locations of import specifiers, including dynamic import and import meta handling.



A very small single JS file (4KiB gzipped) that includes inlined Web Assembly for very fast source analysis of ECMAScript module syntax only.



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](#benchmarks) for more info._



> [Built with](https://github.com/guybedford/es-module-lexer/blob/main/chompfile.toml) [Chomp](https://chompbuild.com/)



### Usage



```

npm install es-module-lexer

```



For use in CommonJS:



```js

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



(async () => {

  // either await init, or call parse asynchronously

  // this is necessary for the Web Assembly boot

  await init;



  const source = 'export var p = 5';

  const [imports, exports] = parse(source);

  

  // Returns "p"

  source.slice(exports[0].s, exports[0].e);

  // Returns "p"

  source.slice(exports[0].ls, exports[0].le);

})();

```



An ES module version is also available:



```js

import { init, parse } from 'es-module-lexer';



(async () => {

  await init;



  const source = `

    import { name } from 'mod\\u1011';

    import json from './json.json' assert { type: 'json' }

    export var p = 5;

    export function q () {



    };

    export { x as 'external name' } from 'external';



    // Comments provided to demonstrate edge cases

    import /*comment!*/ (  'asdf', { assert: { type: 'json' }});

    import /*comment!*/.meta.asdf;

  `;



  const [imports, exports] = parse(source, 'optional-sourcename');



  // Returns "modထ"

  imports[0].n

  // Returns "mod\u1011"

  source.slice(imports[0].s, imports[0].e);

  // "s" = start

  // "e" = end



  // Returns "import { name } from 'mod'"

  source.slice(imports[0].ss, imports[0].se);

  // "ss" = statement start

  // "se" = statement end



  // Returns "{ type: 'json' }"

  source.slice(imports[1].a, imports[1].se);

  // "a" = assert, -1 for no assertion



  // Returns "external"

  source.slice(imports[2].s, imports[2].e);



  // Returns "p"

  source.slice(exports[0].s, exports[0].e);

  // Returns "p"

  source.slice(exports[0].ls, exports[0].le);

  // Returns "q"

  source.slice(exports[1].s, exports[1].e);

  // Returns "q"

  source.slice(exports[1].ls, exports[1].le);

  // Returns "'external name'"

  source.slice(exports[2].s, exports[2].e);

  // Returns -1

  exports[2].ls;

  // Returns -1

  exports[2].le;



  // Dynamic imports are indicated by imports[2].d > -1

  // In this case the "d" index is the start of the dynamic import bracket

  // Returns true

  imports[2].d > -1;



  // Returns "asdf" (only for string literal dynamic imports)

  imports[2].n

  // Returns "import /*comment!*/ (  'asdf', { assert: { type: 'json' } })"

  source.slice(imports[3].ss, imports[3].se);

  // Returns "'asdf'"

  source.slice(imports[3].s, imports[3].e);

  // Returns "(  'asdf', { assert: { type: 'json' } })"

  source.slice(imports[3].d, imports[3].se);

  // Returns "{ assert: { type: 'json' } }"

  source.slice(imports[3].a, imports[3].se - 1);



  // For non-string dynamic import expressions:

  // - n will be undefined

  // - a is currently -1 even if there is an assertion

  // - e is currently the character before the closing )



  // For nested dynamic imports, the se value of the outer import is -1 as end tracking does not

  // currently support nested dynamic immports



  // import.meta is indicated by imports[3].d === -2

  // Returns true

  imports[4].d === -2;

  // Returns "import /*comment!*/.meta"

  source.slice(imports[4].s, imports[4].e);

  // ss and se are the same for import meta

})();

```



### CSP asm.js Build



The default version of the library uses Wasm and (safe) eval usage for performance and a minimal footprint.



Neither of these represent security escalation possibilities since there are no execution string injection vectors, but that can still violate existing CSP policies for applications.



For a version that works with CSP eval disabled, use the `es-module-lexer/js` build:



```js

import { parse } from 'es-module-lexer/js';

```



Instead of Web Assembly, this uses an asm.js build which is almost as fast as the Wasm version ([see benchmarks below](#benchmarks)).



### Escape Sequences



To handle escape sequences in specifier strings, the `.n` field of imported specifiers will be provided where possible.



For dynamic import expressions, this field will be empty if not a valid JS string.



### Facade Detection



Facade modules that only use import / export syntax can be detected via the third return value:



```js

const [,, facade] = parse(`

  export * from 'external';

  import * as ns from 'external2';

  export { a as b } from 'external3';

  export { ns };

`);

facade === true;

```



### ESM Detection



Modules that uses ESM syntaxes can be detected via the fourth return value:



```js

const [,,, hasModuleSyntax] = parse(`

  export {}

`);

hasModuleSyntax === true;

```



Dynamic imports are ignored since they can be used in Non-ESM files.



```js

const [,,, hasModuleSyntax] = parse(`

  import('./foo.js')

`);

hasModuleSyntax === false;

```



### Environment Support



Node.js 10+, and [all browsers with Web Assembly support](https://caniuse.com/#feat=wasm).



### 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:



```js

// 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 for a high spec machine:



#### Wasm Build



```

Module load time

> 5ms

Cold Run, All Samples

test/samples/*.js (3123 KiB)

> 18ms



Warm Runs (average of 25 runs)

test/samples/angular.js (739 KiB)

> 3ms

test/samples/angular.min.js (188 KiB)

> 1ms

test/samples/d3.js (508 KiB)

> 3ms

test/samples/d3.min.js (274 KiB)

> 2ms

test/samples/magic-string.js (35 KiB)

> 0ms

test/samples/magic-string.min.js (20 KiB)

> 0ms

test/samples/rollup.js (929 KiB)

> 4.32ms

test/samples/rollup.min.js (429 KiB)

> 2.16ms



Warm Runs, All Samples (average of 25 runs)

test/samples/*.js (3123 KiB)

> 14.16ms

```



#### JS Build (asm.js)



```

Module load time

> 2ms

Cold Run, All Samples

test/samples/*.js (3123 KiB)

> 34ms



Warm Runs (average of 25 runs)

test/samples/angular.js (739 KiB)

> 3ms

test/samples/angular.min.js (188 KiB)

> 1ms

test/samples/d3.js (508 KiB)

> 3ms

test/samples/d3.min.js (274 KiB)

> 2ms

test/samples/magic-string.js (35 KiB)

> 0ms

test/samples/magic-string.min.js (20 KiB)

> 0ms

test/samples/rollup.js (929 KiB)

> 5ms

test/samples/rollup.min.js (429 KiB)

> 3.04ms



Warm Runs, All Samples (average of 25 runs)

test/samples/*.js (3123 KiB)

> 17.12ms

```



### Building



This project uses [Chomp](https://chompbuild.com) for building.



With Chomp installed, download the WASI SDK 12.0 from https://github.com/WebAssembly/wasi-sdk/releases/tag/wasi-sdk-12.



- [Linux](https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-12/wasi-sdk-12.0-linux.tar.gz)

- [Windows (MinGW)](https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-12/wasi-sdk-12.0-mingw.tar.gz)

- [macOS](https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-12/wasi-sdk-12.0-macos.tar.gz)



Locate the WASI-SDK as a sibling folder, or customize the path via the `WASI_PATH` environment variable.



Emscripten emsdk is also assumed to be a sibling folder or via the `EMSDK_PATH` environment variable.



Example setup:



```

git clone https://github.com:guybedford/es-module-lexer

git clone https://github.com/emscripten-core/emsdk

cd emsdk

git checkout 1.40.1-fastcomp

./emsdk install 1.40.1-fastcomp

cd ..

wget https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-12/wasi-sdk-12.0-linux.tar.gz

gunzip wasi-sdk-12.0-linux.tar.gz

tar -xf wasi-sdk-12.0-linux.tar

mv wasi-sdk-12.0-linux.tar wasi-sdk-12.0

cargo install chompbuild

cd es-module-lexer

chomp test

```



For the `asm.js` build, git clone `emsdk` from  is assumed to be a sibling folder as well.



### License



MIT



[actions-image]: https://github.com/guybedford/es-module-lexer/actions/workflows/build.yml/badge.svg

[actions-url]: https://github.com/guybedford/es-module-lexer/actions/workflows/build.yml