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Use new URL(..., import.meta.url) pattern in minimal runtime if EXPORT_ES6 is enabled #26039

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Use new URL(..., import.meta.url) pattern in minimal runtime if EXPORT_ES6 is enabled #26039

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13afe52
Use `new URL` pattern in minimal runtime
valadaptive Jan 4, 2026
6312129
Avoid adding moduleUrl variable
valadaptive Jan 12, 2026
9b19695
Add minimal runtime Vite test
valadaptive Jan 12, 2026
48605d5
Use @parameterized
valadaptive Jan 12, 2026
fbb38c2
Remove unnecessary test
valadaptive Jan 12, 2026
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13 changes: 11 additions & 2 deletions src/postamble_minimal.js
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Original file line number Diff line number Diff line change
Expand Up @@ -182,6 +182,15 @@ var imports = {
};

#if MINIMAL_RUNTIME_STREAMING_WASM_INSTANTIATION
{{{
#if EXPORT_ES6 && !ENVIRONMENT_MAY_BE_AUDIO_WORKLET
const moduleUrl = `new URL('${TARGET_BASENAME}.wasm', import.meta.url)`;
#elif !EXPORT_ES6 || AUDIO_WORKLET
const moduleUrl = `'${TARGET_BASENAME}.wasm'`;
#else
const moduleUrl = `ENVIRONMENT_IS_AUDIO_WORKLET ? '${TARGET_BASENAME}.wasm' : new URL('${TARGET_BASENAME}.wasm', import.meta.url)`;
#endif
}}}
// https://caniuse.com/#feat=wasm and https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/instantiateStreaming
#if MIN_SAFARI_VERSION < 150000 || ENVIRONMENT_MAY_BE_NODE
#if ASSERTIONS && !WASM2JS
Expand All @@ -197,13 +206,13 @@ instantiatePromise =
// Node's fetch API cannot be used for local files, so we cannot use instantiateStreaming
&& !ENVIRONMENT_IS_NODE
#endif
? WebAssembly.instantiateStreaming(fetch('{{{ TARGET_BASENAME }}}.wasm'), imports)
? WebAssembly.instantiateStreaming(fetch({{{ moduleUrl }}}), imports)
: WebAssembly.instantiate(Module['wasm'], imports)).then((output) => {
#else
#if AUDIO_WORKLET
instantiatePromise =
#endif
WebAssembly.instantiateStreaming(fetch('{{{ TARGET_BASENAME }}}.wasm'), imports).then((output) => {
WebAssembly.instantiateStreaming(fetch({{{ moduleUrl }}}), imports).then((output) => {
#endif

#else // Non-streaming instantiation
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15 changes: 15 additions & 0 deletions test/test_browser.py
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Expand Up @@ -5563,6 +5563,21 @@ def test_vite(self):
self.run_process(shared.get_npm_cmd('vite') + ['build'])
self.run_browser('dist/index.html', '/report_result?exit:0')

@also_with_threads
@requires_dev_dependency('vite')
def test_vite_minimal_runtime(self):
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@sbc100 sbc100 Jan 12, 2026

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Can you confirm this test fails before this PR, and passes after?

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@valadaptive valadaptive Jan 12, 2026

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Manually confirmed the (newly @parameterized) test fails without the changes made in this PR.

copytree(test_file('vite'), '.')
self.compile_btest('hello_world.c', [
'-sEXIT_RUNTIME',
'-sMINIMAL_RUNTIME',
'-sMINIMAL_RUNTIME_STREAMING_WASM_INSTANTIATION',
'-sEXPORT_ES6',
'-sENVIRONMENT=web',
'-o',
'hello.mjs'])
self.run_process(shared.get_npm_cmd('vite') + ['build'])
self.run_browser('dist/index.html', '/report_result?exit:0')
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@sbc100 sbc100 Jan 12, 2026

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Rather than adding a new test here you should be able to using @parameterize.

Something like:

  @parameterized({                                                                                        
    '': ([],),                                                                                                                          
    'minimal': (['-sMINIMAL_RUNTIME', '-sMINIMAL_RUNTIME_STREAMING_WASM_INSTANTIATION'],),                                        
  })  
  def test_vite_minimal_runtime(self, args):

(then add args to the compile_btest arguments).

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@valadaptive valadaptive Jan 12, 2026

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Updated.


@also_with_threads
@requires_dev_dependency('rollup')
def test_rollup(self):
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12 changes: 12 additions & 0 deletions test/test_other.py
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Expand Up @@ -1702,6 +1702,18 @@ def test_minimal_runtime_errors(self):
expected = 'emcc: error: MINIMAL_RUNTIME is not compatible with --preload-file'
self.assert_fail([EMCC, test_file('hello_world.c'), '-sMINIMAL_RUNTIME', '--preload-file', 'foo'], expected)

def test_minimal_runtime_esm_streaming_instantiation(self):
self.run_process([EMCC, '-o', 'test.mjs',
'-sMINIMAL_RUNTIME',
'-sMINIMAL_RUNTIME_STREAMING_WASM_INSTANTIATION',
'-sEXPORT_ES6',
'-sENVIRONMENT=web',
test_file('hello_world.c')])

src = read_file('test.mjs')
# Verify that the generated code uses import.meta.url for the wasm URL
self.assertContained("new URL('test.wasm', import.meta.url)", src)
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@sbc100 sbc100 Jan 9, 2026

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We have test_vite in test_browser.py what should be able to not only test that this string exists but that it works in the vite bundler.

IIUC we could add a variant of that test that uses -sMINIMAL_RUNTIME -sMINIMAL_RUNTIME_STREAMING_WASM_INSTANTIATION ?

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@valadaptive valadaptive Jan 12, 2026

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I've added a test for Vite. Should I keep this one as well?

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@sbc100 sbc100 Jan 12, 2026

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Probably not worth keeping this one too

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@valadaptive valadaptive Jan 12, 2026

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Removed it.


def test_export_all_and_exported_functions(self):
# EXPORT_ALL should not export library functions by default.
# This means that to export library function you also need to explicitly
Expand Down