es-module-lexer
Advanced tools
Package description
The es-module-lexer package is designed to perform lexical analysis of JavaScript modules to identify import and export statements. It is particularly useful for tools that need to analyze or transform ES module syntax, such as bundlers, compilers, and code analysis tools.
Lexical Analysis
This feature allows you to perform lexical analysis on a string containing ES module source code. The `parse` function returns two arrays: one for the import statements and one for the export statements found in the source code.
import { init, parse } from 'es-module-lexer';
(async () => {
await init;
const source = `import { a } from 'module-a';`;
const [imports, exports] = parse(source);
console.log(imports);
console.log(exports);
})();Acorn is a small, fast, JavaScript-based JavaScript parser. It provides a simple interface for parsing JavaScript code and generating abstract syntax trees (AST). While es-module-lexer focuses specifically on ES module syntax, Acorn is a more general-purpose parser that can handle a wider range of JavaScript features.
Babel-parser (previously known as @babel/parser) is part of the Babel toolchain and is used to parse JavaScript code into an AST. It is highly configurable and supports experimental language features. Compared to es-module-lexer, babel-parser offers a more comprehensive analysis of the code but may be slower due to its broader scope and feature set.
Cherow is a fast, standards-compliant, self-hosted JavaScript parser with error recovery. It aims to parse according to the ECMAScript specification. Cherow can be seen as an alternative to es-module-lexer with a focus on compliance and error recovery, but it is not limited to module syntax analysis.
Readme
A JS module syntax lexer used in 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 for more info.
npm install es-module-lexer
For use in CommonJS:
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/es-module-lexer.js:
import { init, parse } from 'es-module-lexer/dist/es-module-lexer.js';
(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);
// 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);
})();
Node.js 10+, and all browsers with Web Assembly support.
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.
Benchmarks can be run with npm run bench.
Current results:
Cold Run, All Samples
test/samples/*.js (3057 KiB)
> 25ms
Warm Runs (average of 25 runs)
test/samples/angular.js (719 KiB)
> 5ms
test/samples/angular.min.js (188 KiB)
> 3.2ms
test/samples/d3.js (491 KiB)
> 4.32ms
test/samples/d3.min.js (274 KiB)
> 2.08ms
test/samples/magic-string.js (34 KiB)
> 0ms
test/samples/magic-string.min.js (20 KiB)
> 0ms
test/samples/rollup.js (902 KiB)
> 5.8ms
test/samples/rollup.min.js (429 KiB)
> 3.16ms
Warm Runs, All Samples (average of 25 runs)
test/samples/*.js (3057 KiB)
> 18.04ms
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.
MIT
FAQs
Lexes ES modules returning their import/export metadata
The npm package es-module-lexer receives a total of 16,905,260 weekly downloads. As such, es-module-lexer popularity was classified as popular.
We found that es-module-lexer demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 0 open source maintainers collaborating on the project.
Did you know?
Socket for GitHub automatically highlights issues in each pull request and monitors the health of all your open source dependencies. Discover the contents of your packages and block harmful activity before you install or update your dependencies.
Product
Socket now supports four distinct alert actions instead of the previous two, and alert triaging allows users to override the actions taken for all individual alerts.
Security News
Polyfill.io has been serving malware for months via its CDN, after the project's open source maintainer sold the service to a company based in China.
Security News
OpenSSF is warning open source maintainers to stay vigilant against reputation farming on GitHub, where users artificially inflate their status by manipulating interactions on closed issues and PRs.