feat: add the ability to test all transports

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 +37,64 @@ 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("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) => {