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    0.10.5 • Public • Published

    ES Module Lexer

    Build Status

    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.

    Built with Chomp


    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:

    import { init, parse } from 'es-module-lexer';
    (async () => {
      await init;
      const source = `
        import { name } from 'mod\\u1011';
        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 "modထ"
      // Returns "mod\u1011"
      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, -1 for no assertion
      // Returns "p,q"
      // Dynamic imports are indicated by imports[2].d > -1
      // In this case the "d" index is the start of the dynamic import bracket
      // Returns true
      imports[2].d > -1;
      // Returns "asdf" (only for string literal dynamic imports)
      // Returns "import /*comment!*/ (  'asdf', { assert: { type: 'json' } })"
      source.substring(imports[2].ss, imports[2].se);
      // Returns "'asdf'"
      source.substring(imports[2].s, imports[2].e);
      // Returns "(  'asdf', { assert: { type: 'json' } })"
      source.substring(imports[2].d, imports[2].se);
      // Returns "{ assert: { type: 'json' } }"
      source.substring(imports[2].a, imports[2].se - 1);
      // For non-string dynamic import expressions:
      // - n will be undefined
      // - a is currently -1 even if there is an assertion
      // - e is currently the character before the closing )
      // For nested dynamic imports, the se value of the outer import is -1 as end tracking does not
      // currently support nested dynamic immports
      // 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);
      // ss and se are the same for import meta

    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:

    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).

    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:

    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.

    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.


    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 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


    This project uses Chomp for building.

    With Chomp installed, download the WASI SDK 12.0 from

    Locate the WASI-SDK as a sibling folder, or customize the path via the WASI_PATH environment variable.

    Emscripten emsdk is also assumed to be a sibling folder or via the EMSDK_PATH environment variable.

    Example setup:

    git clone
    git clone
    gunzip wasi-sdk-12.0-linux.tar.gz
    tar -xf wasi-sdk-12.0-linux.tar
    mv wasi-sdk-12.0-linux.tar wasi-sdk-12.0
    cargo install chompbuild
    cd es-module-lexer
    chomp test

    For the asm.js build, git clone emsdk from is assumed to be a sibling folder as well.






    npm i es-module-lexer

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    • guybedford