es-module-lexer/README.md

# ES Module Lexer

[![Build Status][travis-image]][travis-url]

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._

### 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 [imports, exports] = parse('export var p = 5');
  exports[0] === 'p';
})();
```

An ES module version is also available from `dist/lexer.js`:  
Note: This version will be automatically used in rollup/es-dev-server/node (if an es-module project)

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

(async () => {
  await init;

  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;
  `;

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

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

  // Returns "import { a } from 'asdf';"
  source.substring(imports[0].ss, imports[0].se);

  // 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);
})();
```

### 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:

```
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](https://github.com/WebAssembly/binaryen) prior to publish to reduce the Web Assembly footprint.

### License

MIT

[travis-url]: https://travis-ci.org/guybedford/es-module-lexer
[travis-image]: https://travis-ci.org/guybedford/es-module-lexer.svg?branch=master