Support ES6 module syntax in Acorn, Babel and Closure

src/closure-externs/closure-externs.js

@@ -11,8 +11,7 @@
  * The closure_compiler() method in tools/shared.py refers to this file when calling closure.
  */
 
-// Special placeholder for `import.meta` and `await import`.
-var EMSCRIPTEN$IMPORT$META;
+// Special placeholder for `await import` and `await`.
 var EMSCRIPTEN$AWAIT$IMPORT;
 var EMSCRIPTEN$AWAIT;
 

src/parseTools.mjs

@@ -74,18 +74,22 @@ function findIncludeFile(filename, currentDir) {
 export function preprocess(filename) {
   let text = readFile(filename);
   if (EXPORT_ES6) {
-    // `eval`, Terser and Closure don't support module syntax; to allow it,
-    // we need to temporarily replace `import.meta` and `await import` usages
-    // with placeholders during preprocess phase, and back after all the other ops.
-    // See also: `phase_final_emitting` in emcc.py.
-    text = text
-      .replace(/\bimport\.meta\b/g, 'EMSCRIPTEN$IMPORT$META')
-      .replace(/\bawait import\b/g, 'EMSCRIPTEN$AWAIT$IMPORT');
-  }
-  if (MODULARIZE) {
-    // Same for out use of "top-level-await" which is not actually top level
-    // in the case of MODULARIZE.
-    text = text.replace(/\bawait createWasm\(\)/g, 'EMSCRIPTEN$AWAIT(createWasm())');
+    // `vm.runInContext` doesn't support module syntax; to allow it, we need to
+    // temporarily replace `import.meta` usages with placeholders during the JS
+    // compile phase, then in Python we reverse this replacement.
+    // See also: `emscript` in emscripten.py.
+    text = text.replaceAll('import.meta', 'EMSCRIPTEN$IMPORT$META');
+  }
+  if (MODULARIZE && USE_CLOSURE_COMPILER) {
+    // Closure doesn't support "top-level await" which is not actually the top
+    // level in case of MODULARIZE. Temporarily replace `await` usages with
+    // placeholders during preprocess phase, and back after all the other ops.
+    // See also: `fix_js_mangling` in emcc.py.
+    // FIXME: Remove after https://github.com/google/closure-compiler/issues/3835 is fixed.
+    if (EXPORT_ES6) {
+      text = text.replaceAll('await import', 'EMSCRIPTEN$AWAIT$IMPORT');
+    }
+    text = text.replaceAll('await createWasm()', 'EMSCRIPTEN$AWAIT(createWasm())');
   }
   // Remove windows line endings, if any
   text = text.replace(/\r\n/g, '\n');

src/shell.js

@@ -130,7 +130,7 @@ if (ENVIRONMENT_IS_NODE) {
 #endif
 #endif // PTHREADS || WASM_WORKERS
 }
-#endif // ENVIRONMENT_MAY_BE_NODE
+#endif // ENVIRONMENT_MAY_BE_NODE && (EXPORT_ES6 || PTHREADS || WASM_WORKERS)
 
 // --pre-jses are emitted after the Module integration code, so that they can
 // refer to Module (if they choose; they can also define Module)

test/codesize/test_codesize_minimal_esm.json

@@ -1,10 +1,10 @@
 {
-  "a.out.js": 2448,
-  "a.out.js.gz": 1175,
+  "a.out.js": 2418,
+  "a.out.js.gz": 1169,
   "a.out.nodebug.wasm": 75,
   "a.out.nodebug.wasm.gz": 87,
-  "total": 2523,
-  "total_gz": 1262,
+  "total": 2493,
+  "total_gz": 1256,
   "sent": [],
   "imports": [],
   "exports": [

third_party/terser/README.md

@@ -7,7 +7,7 @@ version published in npm.
 This `terser.js` bundle in this directory was built from our fork of terser
 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.
 
 To make changes to this code please submit patches to
 https://github.com/emscripten-core/terser/ and then re-create this bundle

tools/acorn-optimizer.mjs

@@ -1718,6 +1718,7 @@ const params = {
   ecmaVersion: 'latest',
   sourceType: exportES6 ? 'module' : 'script',
   allowAwaitOutsideFunction: true,
+  allowImportExportEverywhere: exportES6,
 };
 if (closureFriendly) {
   const currentComments = [];

tools/building.py

@@ -541,9 +541,21 @@ def version_split(v):
 @ToolchainProfiler.profile()
 def transpile(filename):
   config = {
-    'sourceType': 'script',
-    'presets': ['@babel/preset-env'],
+    'sourceType': 'module' if settings.EXPORT_ES6 else 'script',
+    'presets': [
+      ['@babel/preset-env', {
+        # Avoid transforming `import()` expressions to non-ESM module formats.
+        'exclude': ['@babel/plugin-transform-dynamic-import'],
+      }],
+    ],
     'targets': {},
+    'parserOpts': {
+      # Allow Babel to parse "top-level await" which is not actually the top
+      # level in case of MODULARIZE.
+      'allowAwaitOutsideFunction': True,
+      # Allow Babel to parse "import.meta".
+      'allowImportExportEverywhere': settings.EXPORT_ES6,
+    },
   }
   if settings.MIN_CHROME_VERSION != UNSUPPORTED:
     config['targets']['chrome'] = str(settings.MIN_CHROME_VERSION)
@@ -630,6 +642,14 @@ def closure_compiler(filename, advanced=True, extra_closure_args=None):
 
   args = ['--compilation_level', 'ADVANCED_OPTIMIZATIONS' if advanced else 'SIMPLE_OPTIMIZATIONS']
   args += ['--language_in', 'UNSTABLE']
+  # Make Closure aware of the ES6 module syntax;
+  # i.e. the `import.meta` and `await import` usages
+  if settings.EXPORT_ES6:
+    args += ['--chunk_output_type', 'ES_MODULES']
+    if settings.ENVIRONMENT_MAY_BE_NODE:
+      args += ['--module_resolution', 'NODE']
+      # https://github.com/google/closure-compiler/issues/3740
+      args += ['--jscomp_off=moduleLoad']
   # We do transpilation using babel
   args += ['--language_out', 'NO_TRANSPILE']
   # Tell closure never to inject the 'use strict' directive.

tools/emscripten.py

@@ -427,6 +427,13 @@ def emscript(in_wasm, out_wasm, outfile_js, js_syms, finalize=True, base_metadat
   assert '//FORWARDED_DATA:' in out, 'Did not receive forwarded data in pre output - process failed?'
   glue, forwarded_data = out.split('//FORWARDED_DATA:', 1)
 
+  # 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.
+  if settings.EXPORT_ES6:
+    glue = glue.replace('EMSCRIPTEN$IMPORT$META', 'import.meta')
+
   forwarded_json = json.loads(forwarded_data)
 
   if forwarded_json['warnings']:

tools/link.py

@@ -2082,19 +2082,28 @@ def phase_source_transforms(options):
   save_intermediate('transformed')
 
 
-# Unmangle previously mangled `import.meta` and `await import` references in
+# Unmangle previously mangled `await import` and `await` references in
 # both main code and libraries.
-# See also: `preprocess` in parseTools.js.
+# See also: `preprocess` in parseTools.mjs.
 def fix_js_mangling(js_file):
-  # We don't apply these mangliings except in MODULARIZE/EXPORT_ES6 modes.
-  if not settings.MODULARIZE:
+  # Mangling only takes place under closure in MODULARIZE mode.
+  if not settings.MODULARIZE or not settings.USE_CLOSURE_COMPILER:
     return
 
   src = read_file(js_file)
-  write_file(js_file, src
-             .replace('EMSCRIPTEN$IMPORT$META', 'import.meta')
-             .replace('EMSCRIPTEN$AWAIT$IMPORT', 'await import')
-             .replace('EMSCRIPTEN$AWAIT(', 'await ('))
+
+  if settings.EXPORT_ES6:
+    # Also remove the line containing `export{};`, which is inserted by
+    # Closure to mark the file as an ES6 module.
+    # https://github.com/google/closure-compiler/issues/4084#issuecomment-1505056519
+    # https://github.com/google/closure-compiler/blob/v20260401/src/com/google/javascript/jscomp/ConvertChunksToESModules.java#L111-L113
+    src = src \
+      .replace('EMSCRIPTEN$AWAIT$IMPORT', 'await import') \
+      .replace('export{};\n', '')
+
+  src = src.replace('EMSCRIPTEN$AWAIT(', 'await (')
+
+  write_file(js_file, src)
   save_intermediate('js-mangling')