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# 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