252 lines
6.8 KiB
Markdown
252 lines
6.8 KiB
Markdown
# ES Module Lexer
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[![Build Status][travis-image]][travis-url]
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A JS module syntax lexer used in [es-module-shims](https://github.com/guybedford/es-module-shims).
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Outputs the list of exports and locations of import specifiers, including dynamic import and import meta handling.
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A very small single JS file (4KiB gzipped) that includes inlined Web Assembly for very fast source analysis of ECMAScript module syntax only.
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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.
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_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._
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### Usage
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```
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npm install es-module-lexer
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```
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For use in CommonJS:
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```js
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const { init, parse } = require('es-module-lexer');
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(async () => {
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// either await init, or call parse asynchronously
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// this is necessary for the Web Assembly boot
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await init;
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const [imports, exports] = parse('export var p = 5');
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exports[0] === 'p';
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})();
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```
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An ES module version is also available:
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```js
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import { init, parse } from 'es-module-lexer';
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(async () => {
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await init;
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const source = `
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import { name } from 'mod';
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import json from './json.json' assert { type: 'json' }
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export var p = 5;
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export function q () {
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};
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// Comments provided to demonstrate edge cases
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import /*comment!*/ ('asdf', { assert: { type: 'json' }});
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import /*comment!*/.meta.asdf;
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`;
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const [imports, exports] = parse(source, 'optional-sourcename');
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// Returns "mod"
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imports[0].n
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source.substring(imports[0].s, imports[0].e);
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// "s" = start
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// "e" = end
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// Returns "import { name } from 'mod'"
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source.substring(imports[0].ss, imports[0].se);
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// "ss" = statement start
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// "se" = statement end
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// Returns "{ type: 'json' }"
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source.substring(imports[1].a, imports[1].se);
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// "a" = assert
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// Returns "p,q"
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exports.toString();
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// Dynamic imports are indicated by imports[2].d > -1
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// In this case the "d" index is the start of the dynamic import
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// Returns true
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imports[2].d > -1;
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// Returns "asdf"
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imports[2].n
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// Returns "'asdf'"
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source.substring(imports[2].s, imports[2].e);
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// Returns "import /*comment!*/ ("
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source.substring(imports[2].d, imports[2].s);
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// Returns "import /*comment!*/ ('asdf', { assert: { type: 'json' } })"
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source.substring(imports[2].d, imports[2].se + 1);
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// Returns "{ assert: { type: 'json' } }"
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source.substring(imports[2].a, imports[2].e);
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// ss is the same as d
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// as, ae not used for dynamic imports
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// import.meta is indicated by imports[2].d === -2
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// Returns true
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imports[2].d === -2;
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// Returns "import /*comment!*/.meta"
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source.substring(imports[2].s, imports[2].e);
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})();
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```
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### CSP asm.js Build
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The default version of the library uses Wasm and (safe) eval usage for performance and a minimal footprint.
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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.
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For a version that works with CSP eval disabled, use the `es-module-lexer/js` build:
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```js
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import { parse } from 'es-module-lexer/js';
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```
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Instead of Web Assembly, this uses an asm.js build which is almost as fast as the Wasm version ([see benchmarks below](#benchmarks)).
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### Escape Sequences
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To handle escape sequences in specifier strings, the `.n` field of imported specifiers will be provided where possible.
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For dynamic import expressions, this field will be empty if not a valid JS string.
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### Facade Detection
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Facade modules that only use import / export syntax can be detected via the third return value:
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```js
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const [,, facade] = parse(`
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export * from 'external';
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import * as ns from 'external2';
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export { a as b } from 'external3';
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export { ns };
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`);
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facade === true;
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```
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### Environment Support
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Node.js 10+, and [all browsers with Web Assembly support](https://caniuse.com/#feat=wasm).
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### Grammar Support
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* Token state parses all line comments, block comments, strings, template strings, blocks, parens and punctuators.
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* Division operator / regex token ambiguity is handled via backtracking checks against punctuator prefixes, including closing brace or paren backtracking.
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* Always correctly parses valid JS source, but may parse invalid JS source without errors.
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### Limitations
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The lexing approach is designed to deal with the full language grammar including RegEx / division operator ambiguity through backtracking and paren / brace tracking.
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The only limitation to the reduced parser is that the "exports" list may not correctly gather all export identifiers in the following edge cases:
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```js
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// Only "a" is detected as an export, "q" isn't
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export var a = 'asdf', q = z;
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// "b" is not detected as an export
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export var { a: b } = asdf;
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```
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The above cases are handled gracefully in that the lexer will keep going fine, it will just not properly detect the export names above.
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### Benchmarks
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Benchmarks can be run with `npm run bench`.
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Current results for a high spec machine:
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#### Wasm Build
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```
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Module load time
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> 5ms
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Cold Run, All Samples
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test/samples/*.js (3123 KiB)
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> 20ms
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Warm Runs (average of 25 runs)
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test/samples/angular.js (739 KiB)
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> 2.12ms
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test/samples/angular.min.js (188 KiB)
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> 1ms
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test/samples/d3.js (508 KiB)
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> 3.04ms
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test/samples/d3.min.js (274 KiB)
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> 2ms
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test/samples/magic-string.js (35 KiB)
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> 0ms
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test/samples/magic-string.min.js (20 KiB)
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> 0ms
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test/samples/rollup.js (929 KiB)
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> 4.04ms
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test/samples/rollup.min.js (429 KiB)
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> 2.16ms
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Warm Runs, All Samples (average of 25 runs)
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test/samples/*.js (3123 KiB)
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> 14.4ms
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```
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#### JS Build (asm.js)
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```
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Module load time
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> 2ms
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Cold Run, All Samples
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test/samples/*.js (3123 KiB)
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> 35ms
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Warm Runs (average of 25 runs)
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test/samples/angular.js (739 KiB)
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> 3ms
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test/samples/angular.min.js (188 KiB)
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> 1.08ms
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test/samples/d3.js (508 KiB)
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> 3.04ms
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test/samples/d3.min.js (274 KiB)
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> 2ms
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test/samples/magic-string.js (35 KiB)
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> 0ms
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test/samples/magic-string.min.js (20 KiB)
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> 0ms
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test/samples/rollup.js (929 KiB)
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> 5.04ms
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test/samples/rollup.min.js (429 KiB)
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> 3ms
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Warm Runs, All Samples (average of 25 runs)
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test/samples/*.js (3123 KiB)
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> 17ms
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```
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### Building
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To build download the WASI SDK from https://github.com/WebAssembly/wasi-sdk/releases.
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The Makefile assumes the existence of "wasi-sdk-11.0" and "wabt" (optional) as sibling folders to this project.
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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`.
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On Windows it may be preferable to use the Linux subsystem.
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After the Web Assembly build, the CJS build can be triggered via `npm run build`.
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### License
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MIT
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[travis-url]: https://travis-ci.org/guybedford/es-module-lexer
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[travis-image]: https://travis-ci.org/guybedford/es-module-lexer.svg?branch=master
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