emscripten-core · sbc100 · Apr 25, 2025 · Apr 21, 2025
diff --git a/ChangeLog.md b/ChangeLog.md
@@ -20,6 +20,9 @@ See docs/process.md for more on how version tagging works.
 
 4.0.8 (in development)
 ----------------------
+- Programs built with `-sWASM_WORKERS` no longer generate a separate `.ww.js`
+  file.  This is similar to the change that was already made for pthreads in
+  #21701.  This saves on complexity, code size and network requests (#24163)
 
 4.0.7 - 04/15/25
 ----------------

diff --git a/site/source/docs/tools_reference/settings_reference.rst b/site/source/docs/tools_reference/settings_reference.rst
@@ -2507,13 +2507,9 @@ Default value: false
 WASM_WORKERS
 ============
 
-If 1, enables support for Wasm Workers. Wasm Workers enable applications
+Enables support for Wasm Workers.  Wasm Workers enable applications
 to create threads using a lightweight web-specific API that builds on top
-of Wasm SharedArrayBuffer + Atomics API. When enabled, a new build output
-file a.ww.js will be generated to bootstrap the Wasm Worker JS contexts.
-If 2, enables support for Wasm Workers, but without using a separate a.ww.js
-file on the side. This can simplify deployment of builds, but will have a
-downside that the generated build will no longer be csp-eval compliant.
+of Wasm SharedArrayBuffer + Atomics API.
 [compile+link] - affects user code at compile and system libraries at link.
 
 Default value: 0

diff --git a/src/audio_worklet.js b/src/audio_worklet.js
@@ -140,6 +140,9 @@ function createWasmAudioWorkletProcessor(audioParams) {
 class BootstrapMessages extends AudioWorkletProcessor {
   constructor(arg) {
     super();
+    // Audio worklets need to show up as wasm workers.  This is the way we signal
+    // that.
+    globalThis.name = 'em-ww';
     // Initialize the global Emscripten Module object that contains e.g. the
     // Wasm Module and Memory objects.  After this we are ready to load in the
     // main application JS script, which the main thread will addModule()

diff --git a/src/lib/libwasm_worker.js b/src/lib/libwasm_worker.js
@@ -4,15 +4,6 @@
  * SPDX-License-Identifier: MIT
  */
 
-#if WASM_WORKERS == 2
-// Helpers for _wasmWorkerBlobUrl used in WASM_WORKERS == 2 mode
-{{{
-  const captureModuleArg = () => MODULARIZE ? '' : 'self.Module=d;';
-  const instantiateModule = () => MODULARIZE ? `${EXPORT_NAME}(d);` : '';
-  const instantiateWasm = () => MINIMAL_RUNTIME ? '' : 'd[`instantiateWasm`]=(i,r)=>{var n=new WebAssembly.Instance(d[`wasm`],i);return r(n,d[`wasm`]);};';
-}}}
-#endif
-
 #if WASM_WORKERS
 
 #if !SHARED_MEMORY
@@ -37,18 +28,28 @@
 {{{
   const workerSupportsFutexWait = () => AUDIO_WORKLET ? "typeof AudioWorkletGlobalScope === 'undefined'" : '1';
   const wasmWorkerJs = `
-#if WASM_WORKERS == 2
-    _wasmWorkerBlobUrl
-#elif MINIMAL_RUNTIME
+#if MINIMAL_RUNTIME
 #if ENVIRONMENT_MAY_BE_NODE
-    Module['$wb'] || './${WASM_WORKER_FILE}'
+    Module['js'] || './${TARGET_JS_NAME}'
 #else
-    Module['$wb']
+    Module['js']
 #endif
 #else
-    locateFile('${WASM_WORKER_FILE}')
+    locateFile('${TARGET_JS_NAME}')
 #endif
 `;
+  const wasmWorkerOptions = `{
+#if ENVIRONMENT_MAY_BE_NODE
+  // This is the way that we signal to the node worker that it is hosting
+  // a wasm worker.
+  'workerData': 'em-ww',
+#endif
+#if ENVIRONMENT_MAY_BE_WEB || ENVIRONMENT_MAY_BE_WORKER
+  // This is the way that we signal to the Web Worker that it is hosting
+  // a pthread.
+  'name': 'em-ww',
+#endif
+}`;
 }}}
 
 #endif // ~WASM_WORKERS
@@ -92,9 +93,13 @@ addToLibrary({
   $_wasmWorkerInitializeRuntime: () => {
     let m = Module;
 #if ASSERTIONS
+    assert(m && m['$ww']);
     assert(m['sb'] % 16 == 0);
     assert(m['sz'] % 16 == 0);
 #endif
+#if RUNTIME_DEBUG
+    dbg("wasmWorkerInitializeRuntime $ww:", m['$ww']);
+#endif
 
 #if !MINIMAL_RUNTIME && isSymbolNeeded('$noExitRuntime')
     // Wasm workers basically never exit their runtime
@@ -149,20 +154,9 @@ addToLibrary({
 #endif
   },
 
-#if WASM_WORKERS == 2
-  // In WASM_WORKERS == 2 build mode, we create the Wasm Worker global scope
-  // script from a string bundled in the main application JS file. This
-  // simplifies the number of deployed JS files with the app, but has a downside
-  // that the generated build output will no longer be csp-eval compliant.
-  // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src#unsafe_eval_expressions
-  $_wasmWorkerBlobUrl: "URL.createObjectURL(new Blob(['onmessage=function(d){onmessage=null;d=d.data;{{{ captureModuleArg() }}}{{{ instantiateWasm() }}}importScripts(d.js);{{{ instantiateModule() }}}d.wasm=d.mem=d.js=0;}'],{type:'application/javascript'}))",
-#endif
   _emscripten_create_wasm_worker__deps: [
     '$_wasmWorkers', '$_wasmWorkersID',
     '$_wasmWorkerAppendToQueue', '$_wasmWorkerRunPostMessage',
-#if WASM_WORKERS == 2
-    '$_wasmWorkerBlobUrl',
-#endif
 #if ASSERTIONS
     'emscripten_has_threading_support',
 #endif
@@ -174,7 +168,7 @@ if (ENVIRONMENT_IS_WASM_WORKER
   && !ENVIRONMENT_IS_AUDIO_WORKLET
 #endif
   ) {
-  _wasmWorkers[0] = this;
+  _wasmWorkers[0] = globalThis;
   addEventListener("message", _wasmWorkerAppendToQueue);
 }`,
   _emscripten_create_wasm_worker: (stackLowestAddress, stackSize) => {
@@ -191,28 +185,21 @@ if (ENVIRONMENT_IS_WASM_WORKER
       var p = trustedTypes.createPolicy(
           'emscripten#workerPolicy1', { createScriptURL: (ignored) => {{{ wasmWorkerJs }}}}
       );
-      worker = _wasmWorkers[_wasmWorkersID] = new Worker(p.createScriptURL('ignored'));
+      worker = _wasmWorkers[_wasmWorkersID] = new Worker(p.createScriptURL('ignored'), {{{ wasmWorkerOptions }}});
     } else
 #endif
-    worker = _wasmWorkers[_wasmWorkersID] = new Worker({{{ wasmWorkerJs }}});
+    worker = _wasmWorkers[_wasmWorkersID] = new Worker({{{ wasmWorkerJs }}}, {{{ wasmWorkerOptions }}});
     // Craft the Module object for the Wasm Worker scope:
     worker.postMessage({
       // Signal with a non-zero value that this Worker will be a Wasm Worker,
       // and not the main browser thread.
       '$ww': _wasmWorkersID,
 #if MINIMAL_RUNTIME
       'wasm': Module['wasm'],
-#if ENVIRONMENT_MAY_BE_NODE
-      'js': Module['js'] || './{{{ TARGET_JS_NAME }}}',
-#else
-      'js': Module['js'],
-#endif
-      'mem': wasmMemory,
 #else
       'wasm': wasmModule,
-      'js': Module['mainScriptUrlOrBlob'] || _scriptName,
-      'wasmMemory': wasmMemory,
 #endif
+      'mem': wasmMemory,
       'sb': stackLowestAddress, // sb = stack bottom (lowest stack address, SP points at this when stack is full)
       'sz': stackSize,          // sz = stack size
     });

diff --git a/src/parseTools.mjs b/src/parseTools.mjs
@@ -1081,10 +1081,6 @@ function getPerformanceNow() {
   }
 }
 
-function implicitSelf() {
-  return ENVIRONMENT.includes('node') ? 'self.' : '';
-}
-
 function ENVIRONMENT_IS_MAIN_THREAD() {
   return `(!${ENVIRONMENT_IS_WORKER_THREAD()})`;
 }
@@ -1143,7 +1139,6 @@ addToCompileTimeContext({
   getPerformanceNow,
   getUnsharedTextDecoderView,
   hasExportedSymbol,
-  implicitSelf,
   isSymbolNeeded,
   makeDynCall,
   makeEval,

diff --git a/src/postamble_minimal.js b/src/postamble_minimal.js
@@ -95,7 +95,7 @@ var wasmExports;
 var wasmModule;
 #endif
 
-#if PTHREADS
+#if PTHREADS || WASM_WORKERS
 function loadModule() {
   assignWasmImports();
 #endif
@@ -255,12 +255,12 @@ WebAssembly.instantiate(Module['wasm'], imports).then((output) => {
 #endif // ASSERTIONS || WASM == 2
 );
 
-#if PTHREADS
+#if PTHREADS || WASM_WORKERS
 }
 
-if (!ENVIRONMENT_IS_PTHREAD) {
-  // When running in a pthread we delay module loading untill we have
-  // received the module via postMessage
-  loadModule();
-}
+// When running in a background thread we delay module loading until we have
+#if AUDIO_WORKLET
+if (ENVIRONMENT_IS_AUDIO_WORKLET) loadModule();
+#endif
+{{{ runIfMainThread('loadModule();') }}}
 #endif
diff --git a/src/preamble.js b/src/preamble.js
@@ -248,9 +248,7 @@ function exitRuntime() {
 #if STACK_OVERFLOW_CHECK
   checkStackCookie();
 #endif
-#if PTHREADS
-  if (ENVIRONMENT_IS_PTHREAD) return; // PThreads reuse the runtime from the main thread.
-#endif
+  {{{ runIfWorkerThread('return;') }}} // PThreads reuse the runtime from the main thread.
 #if !STANDALONE_WASM
   ___funcs_on_exit(); // Native atexit() functions
 #endif
@@ -266,9 +264,7 @@ function postRun() {
 #if STACK_OVERFLOW_CHECK
   checkStackCookie();
 #endif
-#if PTHREADS
-  if (ENVIRONMENT_IS_PTHREAD) return; // PThreads reuse the runtime from the main thread.
-#endif
+  {{{ runIfWorkerThread('return;') }}} // PThreads reuse the runtime from the main thread.
 
 #if expectToReceiveOnModule('postRun')
   if (Module['postRun']) {
@@ -823,7 +819,7 @@ async function instantiateAsync(binary, binaryFile, imports) {
 
 #if !WASM_ESM_INTEGRATION
 function getWasmImports() {
-#if PTHREADS
+#if PTHREADS || WASM_WORKERS
   assignWasmImports();
 #endif
 #if ASYNCIFY && (ASSERTIONS || ASYNCIFY == 2)
@@ -1008,8 +1004,8 @@ function getWasmImports() {
   }
 #endif
 
-#if PTHREADS
-  if (ENVIRONMENT_IS_PTHREAD) {
+#if PTHREADS || WASM_WORKERS
+  if ({{{ ENVIRONMENT_IS_WORKER_THREAD() }}}) {
     return new Promise((resolve) => {
       wasmModuleReceived = (module) => {
         // Instantiate from the module posted from the main thread.

diff --git a/src/runtime_debug.js b/src/runtime_debug.js
@@ -193,7 +193,7 @@ var runtimeDebug = true; // Switch to false at runtime to disable logging at the
 // Used by XXXXX_DEBUG settings to output debug messages.
 function dbg(...args) {
   if (!runtimeDebug && typeof runtimeDebug != 'undefined') return;
-#if ENVIRONMENT_MAY_BE_NODE && PTHREADS
+#if ENVIRONMENT_MAY_BE_NODE && (PTHREADS || WASM_WORKERS)
   // Avoid using the console for debugging in multi-threaded node applications
   // See https://github.com/emscripten-core/emscripten/issues/14804
   if (ENVIRONMENT_IS_NODE) {

diff --git a/src/runtime_init_memory.js b/src/runtime_init_memory.js
@@ -12,9 +12,10 @@
 // check for full engine support (use string 'subarray' to avoid closure compiler confusion)
 
 function initMemory() {
-#if PTHREADS
-  if (ENVIRONMENT_IS_PTHREAD) return;
-#endif // PTHREADS
+#if AUDIO_WORKLET
+  if (!ENVIRONMENT_IS_AUDIO_WORKLET)
+#endif
+  {{{ runIfWorkerThread('return') }}}
 
 #if expectToReceiveOnModule('wasmMemory')
   if (Module['wasmMemory']) {

diff --git a/src/runtime_pthread.js b/src/runtime_pthread.js
@@ -21,25 +21,6 @@ var sharedModules = {};
 #endif
 
 if (ENVIRONMENT_IS_PTHREAD) {
-#if !MINIMAL_RUNTIME
-  var wasmModuleReceived;
-#endif
-
-#if ENVIRONMENT_MAY_BE_NODE
-  // Node.js support
-  if (ENVIRONMENT_IS_NODE) {
-    // Create as web-worker-like an environment as we can.
-
-    var parentPort = worker_threads['parentPort'];
-    parentPort.on('message', (msg) => onmessage({ data: msg }));
-
-    Object.assign(globalThis, {
-      self: global,
-      postMessage: (msg) => parentPort.postMessage(msg),
-    });
-  }
-#endif // ENVIRONMENT_MAY_BE_NODE
-
   // Thread-local guard variable for one-time init of the JS state
   var initializedJS = false;
 

diff --git a/src/runtime_shared.js b/src/runtime_shared.js
@@ -21,10 +21,32 @@
 #include "runtime_asan.js"
 #endif
 
+#if PTHREADS || WASM_WORKERS
+#if !MINIMAL_RUNTIME
+var wasmModuleReceived;
+#endif
+
+#if ENVIRONMENT_MAY_BE_NODE
+if (ENVIRONMENT_IS_NODE && {{{ ENVIRONMENT_IS_WORKER_THREAD() }}}) {
+  // Create as web-worker-like an environment as we can.
+  var parentPort = worker_threads['parentPort'];
+  parentPort.on('message', (msg) => global.onmessage?.({ data: msg }));
+  Object.assign(globalThis, {
+    self: global,
+    postMessage: (msg) => parentPort['postMessage'](msg),
+  });
+}
+#endif // ENVIRONMENT_MAY_BE_NODE
+#endif
+
 #if PTHREADS
 #include "runtime_pthread.js"
 #endif
 
+#if WASM_WORKERS
+#include "wasm_worker.js"
+#endif
+
 #if LOAD_SOURCE_MAP
 var wasmSourceMap;
 #include "source_map_support.js"

diff --git a/src/settings.js b/src/settings.js
@@ -1637,13 +1637,9 @@ var USE_SQLITE3 = false;
 // [compile+link] - affects user code at compile and system libraries at link.
 var SHARED_MEMORY = false;
 
-// If 1, enables support for Wasm Workers. Wasm Workers enable applications
+// Enables support for Wasm Workers.  Wasm Workers enable applications
 // to create threads using a lightweight web-specific API that builds on top
-// of Wasm SharedArrayBuffer + Atomics API. When enabled, a new build output
-// file a.ww.js will be generated to bootstrap the Wasm Worker JS contexts.
-// If 2, enables support for Wasm Workers, but without using a separate a.ww.js
-// file on the side. This can simplify deployment of builds, but will have a
-// downside that the generated build will no longer be csp-eval compliant.
+// of Wasm SharedArrayBuffer + Atomics API.
 // [compile+link] - affects user code at compile and system libraries at link.
 var WASM_WORKERS = 0;
 

diff --git a/src/settings_internal.js b/src/settings_internal.js
@@ -134,9 +134,6 @@ var USER_EXPORTS = [];
 // name of the file containing wasm binary, if relevant
 var WASM_BINARY_FILE = '';
 
-// name of the file containing the Wasm Worker *.ww.js, if relevant
-var WASM_WORKER_FILE = '';
-
 // Base URL the source mapfile, if relevant
 var SOURCE_MAP_BASE = '';
 

diff --git a/src/shell.js b/src/shell.js
@@ -102,6 +102,12 @@ if (ENVIRONMENT_IS_PTHREAD) {
 #endif
 #endif
 
+#if WASM_WORKERS
+// The way we signal to a worker that it is hosting a pthread is to construct
+// it with a specific name.
+var ENVIRONMENT_IS_WASM_WORKER = globalThis.name == 'em-ww';
+#endif
+
 #if ENVIRONMENT_MAY_BE_NODE
 if (ENVIRONMENT_IS_NODE) {
 #if EXPORT_ES6
@@ -121,14 +127,13 @@ if (ENVIRONMENT_IS_NODE) {
   // is hosting a pthread.
   ENVIRONMENT_IS_PTHREAD = ENVIRONMENT_IS_WORKER && worker_threads['workerData'] == 'em-pthread'
 #endif // PTHREADS
+#if WASM_WORKERS
+  ENVIRONMENT_IS_WASM_WORKER = ENVIRONMENT_IS_WORKER && worker_threads['workerData'] == 'em-ww'
+#endif
 #endif // PTHREADS || WASM_WORKERS
 }
 #endif // ENVIRONMENT_MAY_BE_NODE
 
-#if WASM_WORKERS
-var ENVIRONMENT_IS_WASM_WORKER = !!Module['$ww'];
-#endif
-
 // --pre-jses are emitted after the Module integration code, so that they can
 // refer to Module (if they choose; they can also define Module)
 {{{ preJS() }}}