diff --git a/client_js/CHANGELOG.md b/client_js/CHANGELOG.md
index ef656f0..fa5493c 100644
--- a/client_js/CHANGELOG.md
+++ b/client_js/CHANGELOG.md
@@ -1,5 +1,11 @@
 # CHANGELOG
 
+## 0.3.1
+
+### Features
+
+- Adds optional `createWebSocket` factory function for customizing WebSocket creation.
+
 ## 0.3.0
 
 ### Features
diff --git a/client_js/package.json b/client_js/package.json
index 4e5009a..c746869 100644
--- a/client_js/package.json
+++ b/client_js/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@topical/core",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "description": "Vanilla Topical client.",
   "main": "dist/index.js",
   "typings": "dist/index.d.ts",
diff --git a/client_js/src/index.ts b/client_js/src/index.ts
index ba98b6a..fbb53ff 100644
--- a/client_js/src/index.ts
+++ b/client_js/src/index.ts
@@ -1 +1,7 @@
-export { default as Socket, Params, ParamsInput, SocketState } from "./socket";
+export {
+  default as Socket,
+  Params,
+  ParamsInput,
+  SocketState,
+  WebSocketFactory,
+} from "./socket";
diff --git a/client_js/src/socket.ts b/client_js/src/socket.ts
index 1794729..6b81766 100644
--- a/client_js/src/socket.ts
+++ b/client_js/src/socket.ts
@@ -50,6 +50,8 @@ function topicKey(topic: string[], params: Params): string {
   return `${topic.map(encodeURIComponent).join("/")}?${sortedParams}`;
 }
 
+export type WebSocketFactory = (url: string) => WebSocket;
+
 export default class Socket {
   private socket: WebSocket;
   private closed = false;
@@ -61,15 +63,20 @@ export default class Socket {
   // Maps aliased channel IDs to their target channel IDs
   private aliases: Record<number, number> = {};
   private listeners: ((state: SocketState) => void)[] = [];
+  private createWebSocket: WebSocketFactory;
 
-  constructor(private readonly url: string) {
+  constructor(
+    private readonly url: string,
+    createWebSocket?: WebSocketFactory,
+  ) {
+    this.createWebSocket = createWebSocket ?? ((url) => new WebSocket(url));
     this.socket = this.open();
   }
 
   private open() {
     this.closed = false;
     this.setState("connecting");
-    const socket = new WebSocket(this.url);
+    const socket = this.createWebSocket(this.url);
     socket.addEventListener("open", this.handleSocketOpen);
     socket.addEventListener("error", this.handleSocketError);
     socket.addEventListener("message", this.handleSocketMessage);