From 98aaa6a01f02ab0bb71418691ca25abaa4c1817f Mon Sep 17 00:00:00 2001
From: SudhansuBandha <sudhansu9.vssut@gmail.com>
Date: Tue, 7 Jul 2026 07:43:55 +0530
Subject: [PATCH] worker: populate BroadcastChannel MessageEvent source

Populate the MessageEvent source property with the sender's
worker thread ID for BroadcastChannel messages
originating from worker threads.

Fixes: https://github.com/nodejs/node/issues/59053
Signed-off-by: Sudhansu Bandha <bandhasudhansu@gmail.com>
---
 lib/internal/worker/io.js                     |  9 ++++--
 test/parallel/test-worker-broadcastchannel.js | 31 ++++++++++++++++++-
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/lib/internal/worker/io.js b/lib/internal/worker/io.js
index 786ee36c1927fa..4182538561fb9e 100644
--- a/lib/internal/worker/io.js
+++ b/lib/internal/worker/io.js
@@ -48,6 +48,7 @@ const {
 } = internalBinding('messaging');
 const {
   getEnvMessagePort,
+  threadId,
 } = internalBinding('worker');
 
 const { Readable, Writable } = require('stream');
@@ -346,7 +347,7 @@ function receiveMessageOnPort(port) {
 }
 
 function onMessageEvent(type, data) {
-  this.dispatchEvent(lazyMessageEvent(type, { data }));
+  this.dispatchEvent(lazyMessageEvent(type, { data: data.value, source: data.source }));
 }
 
 function isBroadcastChannel(value) {
@@ -419,13 +420,17 @@ class BroadcastChannel extends EventTarget {
    * @returns {void}
    */
   postMessage(message) {
+    const broadcastMessage = {
+      source: threadId,
+      value: message,
+    };
     if (!isBroadcastChannel(this))
       throw new ERR_INVALID_THIS('BroadcastChannel');
     if (arguments.length === 0)
       throw new ERR_MISSING_ARGS('message');
     if (this[kHandle] === undefined)
       throw new DOMException('BroadcastChannel is closed.', 'InvalidStateError');
-    if (this[kHandle].postMessage(message) === undefined)
+    if (this[kHandle].postMessage(broadcastMessage) === undefined)
       throw new DOMException('Message could not be posted.');
   }
 
diff --git a/test/parallel/test-worker-broadcastchannel.js b/test/parallel/test-worker-broadcastchannel.js
index 12535271c15596..89bccd1fbddf19 100644
--- a/test/parallel/test-worker-broadcastchannel.js
+++ b/test/parallel/test-worker-broadcastchannel.js
@@ -147,7 +147,7 @@ assert.throws(() => new BroadcastChannel(), {
   const bc1 = new BroadcastChannel('channel4');
   const bc2 = new BroadcastChannel('channel4');
   bc1.postMessage('some data');
-  assert.strictEqual(receiveMessageOnPort(bc2).message, 'some data');
+  assert.strictEqual(receiveMessageOnPort(bc2).message.value, 'some data');
   assert.strictEqual(receiveMessageOnPort(bc2), undefined);
   bc1.close();
   bc2.close();
@@ -183,3 +183,32 @@ assert.throws(() => new BroadcastChannel(), {
     "BroadcastChannel { name: 'channel5', active: false }"
   );
 }
+
+{
+  const bc = new BroadcastChannel('worker-source');
+
+  new Worker(`
+    const assert = require('assert');
+    const { BroadcastChannel, threadId } = require('worker_threads');
+
+    const bc = new BroadcastChannel('worker-source');
+
+    bc.onmessage = (evt) => {
+      assert.strictEqual(evt.data, 'from-main');
+      assert.strictEqual(evt.source, 0);
+
+      bc.close();
+    };
+
+    bc.postMessage('from-worker');
+  `, { eval: true });
+
+  bc.onmessage = common.mustCall((evt) => {
+    assert.strictEqual(evt.data, 'from-worker');
+
+    assert.ok(evt.source > 0);
+
+    bc.postMessage('from-main');
+    bc.close();
+  });
+}