Support ES6 module syntax in Acorn, Babel and Closure by kleisauke · Pull Request #23730 · emscripten-core/emscripten

kleisauke · 2025-02-23T11:29:17Z

Acorn, Babel and Closure are now made aware of ES6 module syntax
when linking with -sEXPORT_ES6. The previous mangling of top-level
await, import.meta and dynamic import() is tightened to only
mangle top-level await when Closure is enabled, since this is the
only construct not supported there.

test/code_size/hello_webgl_wasm.json

This file is not included when building with `-sMODULARIZE=instance`.

This is an automatic change generated by tools/maint/rebaseline_tests.py. The following (6) test expectation files were updated by running the tests with `--rebaseline`: ``` codesize/test_codesize_minimal_esm.json: 2504 => 2427 [-77 bytes / -3.08%] codesize/test_minimal_runtime_code_size_hello_webgl2_wasm.json: 13199 => 13656 [+457 bytes / +3.46%] codesize/test_minimal_runtime_code_size_hello_webgl2_wasm2js.json: 18537 => 19154 [+617 bytes / +3.33%] codesize/test_minimal_runtime_code_size_hello_webgl2_wasm_singlefile.json: 15132 => 15604 [+472 bytes / +3.12%] codesize/test_minimal_runtime_code_size_hello_webgl_wasm.json: 12737 => 13163 [+426 bytes / +3.34%] codesize/test_minimal_runtime_code_size_hello_webgl_wasm2js.json: 18063 => 18652 [+589 bytes / +3.26%] Average change: +2.24% (-3.08% - +3.46%) ```

This is an automatic change generated by tools/maint/rebaseline_tests.py. The following (1) test expectation files were updated by running the tests with `--rebaseline`: ``` codesize/test_codesize_minimal_esm.json: 2504 => 2474 [-30 bytes / -1.20%] Average change: -1.20% (-1.20% - -1.20%) ```

This is an automatic change generated by tools/maint/rebaseline_tests.py. The following (1) test expectation files were updated by running the tests with `--rebaseline`: ``` codesize/test_codesize_minimal_esm.json: 2494 => 2525 [+31 bytes / +1.24%] Average change: +1.24% (+1.24% - +1.24%) ```

This is an automatic change generated by tools/maint/rebaseline_tests.py. The following (1) test expectation files were updated by running the tests with `--rebaseline`: ``` codesize/test_codesize_minimal_esm.json: 2525 => 2521 [-4 bytes / -0.16%] Average change: -0.16% (-0.16% - -0.16%) ```

This is an automatic change generated by tools/maint/rebaseline_tests.py. The following (1) test expectation files were updated by running the tests with `--rebaseline`: ``` codesize/test_codesize_minimal_esm.json: 2523 => 2520 [-3 bytes / -0.12%] Average change: -0.12% (-0.12% - -0.12%) ```

kleisauke · 2026-04-04T09:48:53Z

PTAL, this would help PR #26384. Though, perhaps PR #26604 should land first, as it would make the changes to strip-node-prefix.mjs‎ and test_other.py unnecessary.

sbc100 · 2026-04-07T17:41:16Z

src/shell.js

  const { createRequire } = await import('node:module');
-  /** @suppress{duplicate} */
-  var require = createRequire(import.meta.url);
+  global.require = createRequire(import.meta.url);


Should we prefer globalThis here?

We use global.Worker below, so aligning with that seemed reasonable (and it saves 4 bytes). Though, it appears to be legacy:
https://nodejs.org/api/globals.html#global

(there was a reason I originally did this, likely something related to Closure, but I can't recall the exact reason anymore)

... looking at commit ef36474, I think that I know the reason behind this. This was likely necessary for the now-removed strip-node-prefix.mjs Babel plugin, which required require() to remain unminified. Let me revert this.

Reverted this with commit af43dd1, which further reduces the codesize expectations of codesize.test_codesize_minimal_esm.

sbc100 · 2026-04-07T17:42:48Z

third_party/terser/README.md

 which lives at: https://github.com/emscripten-core/terser/

-The current patches are stored in the `emscripten_patches_v4.8.0` branch.
+The current patches are stored in the `emscripten_patches_v5.18.2` branch.


Is this a driveby fix? Was this just no updated at some point?

Yep, a drive-by fix, since we're now using Terser 5.18.2.

sbc100 · 2026-04-07T17:45:07Z

tools/emscripten.py

+  # Unmangle previously mangled `import.meta` references in lib*.js.
+  # See also: `LibraryManager.load` in modules.mjs.
+  if settings.EXPORT_ES6:
+    glue = glue.replace('EMSCRIPTEN$IMPORT$META', 'import.meta')


Is there a reason we cannot do the demangling in the JS compiler (i.e. the same place where we do the mangling?)

Ah, good idea. Let me try to move this to the JS compiler, I think that would work.

... that didn't work because the library contents (e.g. libpthread.js where import.meta is used) are streamed to stdout.

emscripten/src/jsifier.mjs

Lines 394 to 401 in 426fb2d

let outputHandle = process.stdout;

if (outputFile) {

outputHandle = await fs.open(outputFile, 'w');

}

async function writeOutput(str) {

await outputHandle.write(str + '\n');

}

So, this is probably the only place where the full file contents is available and where you can a single replace in one go. Added a comment with cf6417c instead.

Maybe we could run this transform over all of the libraryItems before we call writeOutput?

sbc100 · 2026-04-07T17:48:47Z

src/modules.mjs

+      // we reverse this replacement.
+      // See also: `emscript` in emscripten.py.
+      contents = contents.replace(/\bimport\.meta\b/g, 'EMSCRIPTEN$IMPORT$META');
+    }


Why does this need to move from preprocess to here?

Its a little confusing to have this mangling done in two different places.

Moved back to preprocess with commit 5c2f12b (and also updated to use replaceAll in commit 0b218f2 to avoid regex).

This reverts commit ef36474.

This is an automatic change generated by tools/maint/rebaseline_tests.py. The following (1) test expectation files were updated by running the tests with `--rebaseline`: ``` codesize/test_codesize_minimal_esm.json: 2520 => 2493 [-27 bytes / -1.07%] Average change: -1.07% (-1.07% - -1.07%) ```

sbc100 · 2026-04-07T20:17:39Z

tools/emscripten.py

+  # Unmangle previously mangled `import.meta` references in lib*.js.
+  # See also: `preprocess` in parseTools.mjs.
+  # Note: this needs to be done here because this is the only stage
+  # where the complete file contents are available.


But why can't this be done in fix_js_mangling?

sbc100 reviewed Feb 23, 2025

View reviewed changes

test/code_size/hello_webgl_wasm.json Outdated Show resolved Hide resolved

kleisauke force-pushed the es6-acorn-closure branch from f53a92d to c6f028c Compare February 25, 2025 10:25

kleisauke force-pushed the es6-acorn-closure branch from c6f028c to d8831e2 Compare March 6, 2025 16:36

kleisauke added 5 commits January 10, 2026 21:41

Cherry-pick 234940f

ea0ed86

Fix test failure in core2.test_modularize_closure_pre

decb8f8

Fix test failure in core2.test_pthread_proxying_modularize

908c47c

Remove redundant code

985f4dc

This file is not included when building with `-sMODULARIZE=instance`.

kleisauke force-pushed the es6-acorn-closure branch from d8831e2 to 254ed04 Compare January 10, 2026 20:46

kleisauke added 16 commits January 11, 2026 10:30

Fix test failure in browser.test_cross_origin_es6

e194143

Ensure 2-space indent

047b852

Fix test failure in browser_2gb.test_source_phase_imports

b077e9c

Avoid dependence on PR emscripten-core#23261

b7c92f5

Closure doesn't support top-level await

1fbc5ca

Allow Babel to parse top-level await

dbf6e5e

Merge branch 'main' into es6-acorn-closure

e9f3e23

Merge branch 'main' into es6-acorn-closure

31cf27b

Revise comments

f2fbf66

Allow Babel to parse import.meta

6ac9b01

Ensure import() expressions are not Babeled

083405a

Ensure Closure doesn't minify require()

ef36474

globalThis -> global

711efc1

kleisauke changed the title ~~Support ES6 module syntax in Acorn and Closure~~ Support ES6 module syntax in Acorn, Babel and Closure Jan 19, 2026

kleisauke marked this pull request as ready for review January 19, 2026 10:42

vogel76 mentioned this pull request Mar 3, 2026

Eliminate createRequire/require from EXPORT_ES6 output #26384

Open

kleisauke added 2 commits April 4, 2026 11:35

Add comment regarding export{};

5fd7830

Merge branch 'main' into es6-acorn-closure

06d1884

kleisauke added 2 commits April 5, 2026 12:01

modules.js was renamed to modules.mjs

bd18866

Merge branch 'main' into es6-acorn-closure

2d6ba19

sbc100 reviewed Apr 7, 2026

View reviewed changes

kleisauke added 6 commits April 7, 2026 20:05

Revert "Ensure Closure doesn't minify require()"

af43dd1

This reverts commit ef36474.

Restore the mangling logic to its original location

5c2f12b

Prefer use of replaceAll

0b218f2

Add comment

cf6417c

parseTools.js was renamed to parseTools.mjs

f74a9b7

sbc100 reviewed Apr 7, 2026

View reviewed changes

	let outputHandle = process.stdout;
	if (outputFile) {
	outputHandle = await fs.open(outputFile, 'w');
	}

	async function writeOutput(str) {
	await outputHandle.write(str + '\n');
	}

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