ES Module Lexer

A JS module syntax lexer used in es-module-shims.

A very small single JS file (4KiB gzipped) that includes inlined Web Assembly to very fast source analysis for ES modules only.

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

Comprehensively handles the JS language grammar while remaining small and fast - can parse 2MB of JavaScript in under 30ms from a completely cold start, and in just 20ms after a few runs, see benchmarks for more info.

Usage

npm install es-module-lexer

import analyze from 'es-module-lexer';

// Note: Parsing error messages thrown are not user friendly
//       and only provide stack information in the lexer itself.
//       It is recommended to always catch and fall back to an alternative
//       parser for proper error output.

const source = `
  import { a } from 'asdf';
  export var p = 5;
  export function q () {

  };

  // Comments provided to demonstrate edge cases
  import /*comment!*/ ('asdf');
  import /*comment!*/.meta.asdf;
`;

try {
  var [imports, exports] = analyze(source);  
}
catch (e) {
  console.log('Parsing failure');
}

// Returns "asdf"
source.substring(imports[0].s, imports[0].e);

// Returns "p,q"
exports.toString();

// Dynamic imports are indicated by imports[1].d > -1
// In this case the "d" index is the start of the dynamic import
// Returns true
imports[1].d > -1;

// Returns "'asdf'"
source.substring(imports[1].s, imports[1].e);
// Returns "import /*comment!*/ ("
source.substring(imports[1].d, imports[1].s);

// import.meta is indicated by imports[2].d === -2
// Returns true
imports[2].d === -2;
// Returns "import /*comment!*/.meta"
source.substring(imports[2].s, imports[2].e);

Benchmarks

Benchmarks can be run with npm run bench.

Current results:

Module load time
> 6ms
Cold Run, All Samples
test/samples/*.js (2150 KiB)
> 29ms

Warm Runs (average of 25 runs)
test/samples/d3.js (491 KiB)
> 5.6ms
test/samples/d3.min.js (274 KiB)
> 3.44ms
test/samples/magic-string.js (34 KiB)
> 0.36ms
test/samples/magic-string.min.js (20 KiB)
> 0.04ms
test/samples/rollup.js (902 KiB)
> 9.24ms
test/samples/rollup.min.js (429 KiB)
> 5.24ms

Warm Runs, All Samples (average of 25 runs)
test/samples/*.js (2150 KiB)
> 24.8ms

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.

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.

License

MIT