feat: add the ability to test all transports

When setting the `tryAllTransports` option to `true`, if the first
transport (usually, HTTP long-polling) fails, then the other transports
will be tested too.

This is useful in two cases:

> when HTTP long-polling is disabled on the server, or if CORS fails

Related:

- https://github.com/socketio/engine.io-client/issues/575
- https://github.com/socketio/socket.io-client/issues/1448

> when WebSocket is tested first (`transports: ["websocket", "polling"])

Related:

- https://github.com/socketio/engine.io-client/issues/714
- https://github.com/socketio/socket.io-client/issues/1599

The only potential downside is that the connection attempt could take
more time in case of failure, as there have been reports of WebSocket
connection errors taking several seconds before being detected (that's
one reason for using HTTP long-polling first). That's why the option
defaults to `false` for now.

lib/socket.ts

@@ -83,6 +83,19 @@ export interface SocketOptions {
    */
   transports?: string[];
 
+  /**
+   * Whether all the transports should be tested, instead of just the first one.
+   *
+   * If set to `true`, the client will first try to connect with HTTP long-polling, and then with WebSocket in case of
+   * failure, and finally with WebTransport if the previous attempts have failed.
+   *
+   * If set to `false` (default), if the connection with HTTP long-polling fails, then the client will not test the
+   * other transports and will abort the connection.
+   *
+   * @default false
+   */
+  tryAllTransports?: boolean;
+
   /**
    * If true and if the previous websocket connection to the server succeeded,
    * the connection attempt will bypass the normal upgrade process and will
@@ -916,6 +929,17 @@ export class Socket extends Emitter<
   private onError(err: Error) {
     debug("socket error %j", err);
     Socket.priorWebsocketSuccess = false;
+
+    if (
+      this.opts.tryAllTransports &&
+      this.transports.length > 1 &&
+      this.readyState === "opening"
+    ) {
+      debug("trying next transport");
+      this.transports.shift();
+      return this.open();
+    }
+
     this.emitReserved("error", err);
     this.onClose("transport error", err);
   }

lib/transports/websocket.ts

@@ -17,6 +17,10 @@ const isReactNative =
   typeof navigator.product === "string" &&
   navigator.product.toLowerCase() === "reactnative";
 
+/**
+ * @see https://developer.mozilla.org/en-US/docs/Web/API/WebSocket
+ * @see https://caniuse.com/mdn-api_websocket
+ */
 export class WS extends Transport {
   private ws: any;
 
@@ -37,11 +41,6 @@ export class WS extends Transport {
   }
 
   override doOpen() {
-    if (!this.check()) {
-      // let probe timeout
-      return;
-    }
-
     const uri = this.uri();
     const protocols = this.opts.protocols;
 
@@ -189,14 +188,4 @@ export class WS extends Transport {
 
     return this.createUri(schema, query);
   }
-
-  /**
-   * Feature detection for WebSocket.
-   *
-   * @return {Boolean} whether this transport is available.
-   * @private
-   */
-  private check() {
-    return !!WebSocket;
-  }
 }

lib/transports/webtransport.ts

@@ -9,6 +9,10 @@ import debugModule from "debug"; // debug()
 
 const debug = debugModule("engine.io-client:webtransport"); // debug()
 
+/**
+ * @see https://developer.mozilla.org/en-US/docs/Web/API/WebTransport
+ * @see https://caniuse.com/webtransport
+ */
 export class WT extends Transport {
   private transport: any;
   private writer: any;
@@ -18,15 +22,15 @@ export class WT extends Transport {
   }
 
   protected doOpen() {
-    // @ts-ignore
-    if (typeof WebTransport !== "function") {
-      return;
+    try {
+      // @ts-ignore
+      this.transport = new WebTransport(
+        this.createUri("https"),
+        this.opts.transportOptions[this.name]
+      );
+    } catch (err) {
+      return this.emitReserved("error", err);
     }
-    // @ts-ignore
-    this.transport = new WebTransport(
-      this.createUri("https"),
-      this.opts.transportOptions[this.name]
-    );
 
     this.transport.closed
       .then(() => {

test/socket.js

@@ -37,6 +37,66 @@ describe("Socket", function () {
     });
   });
 
+  it("should connect with the 2nd transport if tryAllTransports is `true` (polling)", (done) => {
+    const socket = new Socket({
+      transports: ["websocket", "polling"],
+      transportOptions: {
+        websocket: {
+          query: {
+            deny: 1,
+          },
+        },
+      },
+      tryAllTransports: true,
+    });
+
+    socket.on("open", () => {
+      expect(socket.transport.name).to.eql("polling");
+      socket.close();
+      done();
+    });
+  });
+
+  it("should connect with the 2nd transport if tryAllTransports is `true` (websocket)", (done) => {
+    const socket = new Socket({
+      transports: ["polling", "websocket"],
+      transportOptions: {
+        polling: {
+          query: {
+            deny: 1,
+          },
+        },
+      },
+      tryAllTransports: true,
+    });
+
+    socket.on("open", () => {
+      expect(socket.transport.name).to.eql("websocket");
+      socket.close();
+      done();
+    });
+  });
+
+  it("should not connect with the 2nd transport if tryAllTransports is `false`", (done) => {
+    const socket = new Socket({
+      transports: ["polling", "websocket"],
+      transportOptions: {
+        polling: {
+          query: {
+            deny: 1,
+          },
+        },
+      },
+    });
+
+    socket.on("error", (err) => {
+      expect(err.message).to.eql(
+        useFetch ? "fetch read error" : "xhr poll error"
+      );
+      done();
+    });
+  });
+
   describe("fake timers", function () {
     before(function () {
       if (isIE11 || isAndroid || isEdge || isIPad) {

test/support/hooks.js

@@ -19,6 +19,12 @@ exports.mochaHooks = {
     engine = attach(httpServer, {
       pingInterval: 500,
       maxHttpBufferSize: 100,
+      allowRequest: (req, fn) => {
+        const denyRequest = new URL(`http://${req.url}`).searchParams.has(
+          "deny"
+        );
+        fn(null, !denyRequest);
+      },
     });
 
     rollup(rollupConfig).then(async (bundle) => {