feat: add the "closeOnBeforeunload" option

Since [1], the socket is now closed when receiving the "beforeunload"
event in the browser.

This change was meant to fix a discrepancy between Chrome and Firefox
when the user reloads/closes a browser tab: Firefox would close the
connection (and emit a "disconnect" event, at the Socket.IO level), but
not Chrome (see [2]).

But it also closes the connection when there is another "beforeunload"
handler, for example when the user is prompted "are you sure you want
to leave this page?".

Note: calling "stopImmediatePropagation()" was a possible workaround:

```js
window.addEventListener('beforeunload', (event) => {
  event.preventDefault();
  event.stopImmediatePropagation();
  event.returnValue = 'are you sure you want to leave this page?';
});
```

This commit adds a "closeOnBeforeunload" option, which controls whether
a handler is registered for the "beforeunload" event.

Syntax:

```js
const socket = require('engine.io-client')('ws://localhost', {
  closeOnBeforeunload: false // defaults to true
});
```

[1]: ed48b5d
[2]: socketio/socket.io#3639

Related:

- #661
- #658
- socketio/socket.io-client#1451

Reference: https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event

README.md

@@ -267,6 +267,7 @@ Exposed as `eio` in the browser standalone build.
       - `requestTimeout` (`Number`): Timeout for xhr-polling requests in milliseconds (`0`)
     - **Websocket-only options**
       - `protocols` (`Array`): a list of subprotocols (see [MDN reference](https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API/Writing_WebSocket_servers#Subprotocols))
+      - `closeOnBeforeunload` (`Boolean`): whether to silently close the connection when the [`beforeunload`](https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event) event is emitted in the browser (defaults to `true`)
 - `send`
     - Sends a message to the server
     - **Parameters**

lib/socket.js

@@ -70,7 +70,8 @@ class Socket extends Emitter {
         perMessageDeflate: {
           threshold: 1024
         },
-        transportOptions: {}
+        transportOptions: {},
+        closeOnBeforeunload: true
       },
       opts
     );
@@ -91,17 +92,22 @@ class Socket extends Emitter {
     this.pingTimeoutTimer = null;
 
     if (typeof addEventListener === "function") {
-      addEventListener(
-        "beforeunload",
-        () => {
-          if (this.transport) {
-            // silently close the transport
-            this.transport.removeAllListeners();
-            this.transport.close();
-          }
-        },
-        false
-      );
+      if (this.opts.closeOnBeforeunload) {
+        // Firefox closes the connection when the "beforeunload" event is emitted but not Chrome. This event listener
+        // ensures every browser behaves the same (no "disconnect" event at the Socket.IO level when the page is
+        // closed/reloaded)
+        addEventListener(
+          "beforeunload",
+          () => {
+            if (this.transport) {
+              // silently close the transport
+              this.transport.removeAllListeners();
+              this.transport.close();
+            }
+          },
+          false
+        );
+      }
       if (this.hostname !== "localhost") {
         this.offlineEventListener = () => {
           this.onClose("transport close");