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Editing: 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:

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

(async () => {
  await init;

const source = `
    import { name } from 'mod';
    import json from './json.json' assert { type: 'json' }
    export var p = 5;
    export function q () {

};

// Comments provided to demonstrate edge cases
    import /*comment!*/ ('asdf', { assert: { type: 'json' }});
    import /*comment!*/.meta.asdf;
  `;

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

// Returns "mod"
  imports[0].n
  source.substring(imports[0].s, imports[0].e);
  // "s" = start
  // "e" = end

// Returns "import { name } from 'mod'"
  source.substring(imports[0].ss, imports[0].se);
  // "ss" = statement start
  // "se" = statement end

// Returns "{ type: 'json' }"
  source.substring(imports[1].a, imports[1].se);
  // "a" = assert

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

// Dynamic imports are indicated by imports[2].d > -1
  // In this case the "d" index is the start of the dynamic import
  // Returns true
  imports[2].d > -1;

// Returns "asdf"
  imports[2].n
  // Returns "'asdf'"
  source.substring(imports[2].s, imports[2].e);
  // Returns "import /*comment!*/ ("
  source.substring(imports[2].d, imports[2].s);
  // Returns "import /*comment!*/ ('asdf', { assert: { type: 'json' } })"
  source.substring(imports[2].d, imports[2].se + 1);
  // Returns "{ assert: { type: 'json' } }"
  source.substring(imports[2].a, imports[2].e);
  // ss is the same as d
  // as, ae not used for dynamic imports

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

### 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;
```

### 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)
> 20ms

Warm Runs (average of 25 runs)
test/samples/angular.js (739 KiB)
> 2.12ms
test/samples/angular.min.js (188 KiB)
> 1ms
test/samples/d3.js (508 KiB)
> 3.04ms
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)
> 4.04ms
test/samples/rollup.min.js (429 KiB)
> 2.16ms

Warm Runs, All Samples (average of 25 runs)
test/samples/*.js (3123 KiB)
> 14.4ms
```

#### JS Build (asm.js)

```
Module load time
> 2ms
Cold Run, All Samples
test/samples/*.js (3123 KiB)
> 35ms

Warm Runs (average of 25 runs)
test/samples/angular.js (739 KiB)
> 3ms
test/samples/angular.min.js (188 KiB)
> 1.08ms
test/samples/d3.js (508 KiB)
> 3.04ms
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)
> 5.04ms
test/samples/rollup.min.js (429 KiB)
> 3ms

Warm Runs, All Samples (average of 25 runs)
test/samples/*.js (3123 KiB)
> 17ms
```

### Building

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

The Makefile assumes the existence of "wasi-sdk-11.0" and "wabt" (optional) as sibling folders to this project.

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`.

### 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

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