ES Module Lexer

A JS module syntax lexer used in 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 for more info.

Built with Chomp

Usage

npm install es-module-lexer

For use in CommonJS:

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:

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:

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).

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:

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:

const [,,, hasModuleSyntax] = parse(`
  export {}
`);
hasModuleSyntax === true;

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

const [,,, hasModuleSyntax] = parse(`
  import('./foo.js')
`);
hasModuleSyntax === false;

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 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 for building.

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

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.