feat: add HTTP long-polling implementation based on fetch()

Usage:

```js
import { Socket, transports, Fetch } from "engine.io-client";

transports.polling = Fetch;

const socket = new Socket("https://example.com");
```

Note: tree-shaking unused transports is not currently supported and
will be added later.

Related:

- socketio/socket.io#4980
- #716

.github/workflows/ci.yml

@@ -30,3 +30,7 @@ jobs:
 
       - name: Run tests
         run: npm test
+
+      - name: Run tests with fetch()
+        run: npm run test:node-fetch
+        if: ${{ matrix.node-version == '20' }} # fetch() was added in Node.js v18.0.0 (without experimental flag)

lib/index.ts

@@ -8,3 +8,6 @@ export { transports } from "./transports/index.js";
 export { installTimerFunctions } from "./util.js";
 export { parse } from "./contrib/parseuri.js";
 export { nextTick } from "./transports/websocket-constructor.js";
+
+export { Fetch } from "./transports/polling-fetch.js";
+export { XHR } from "./transports/polling-xhr.js";

lib/transports/index.ts

@@ -1,9 +1,9 @@
-import { Polling } from "./polling.js";
+import { XHR } from "./polling-xhr.js";
 import { WS } from "./websocket.js";
 import { WT } from "./webtransport.js";
 
 export const transports = {
   websocket: WS,
   webtransport: WT,
-  polling: Polling,
+  polling: XHR,
 };

lib/transports/polling-fetch.ts

@@ -0,0 +1,72 @@
+import { Polling } from "./polling.js";
+import { CookieJar, createCookieJar } from "./xmlhttprequest.js";
+
+/**
+ * HTTP long-polling based on `fetch()`
+ *
+ * @see https://developer.mozilla.org/en-US/docs/Web/API/fetch
+ */
+export class Fetch extends Polling {
+  private readonly cookieJar?: CookieJar;
+
+  constructor(opts) {
+    super(opts);
+
+    if (this.opts.withCredentials) {
+      this.cookieJar = createCookieJar();
+    }
+  }
+
+  override doPoll() {
+    this._fetch()
+      .then((res) => {
+        if (!res.ok) {
+          return this.onError("fetch read error", res.status, res);
+        }
+
+        res.text().then((data) => this.onData(data));
+      })
+      .catch((err) => {
+        this.onError("fetch read error", err);
+      });
+  }
+
+  override doWrite(data: string, callback: () => void) {
+    this._fetch(data)
+      .then((res) => {
+        if (!res.ok) {
+          return this.onError("fetch write error", res.status, res);
+        }
+
+        callback();
+      })
+      .catch((err) => {
+        this.onError("fetch write error", err);
+      });
+  }
+
+  private _fetch(data?: string) {
+    const isPost = data !== undefined;
+    const headers = new Headers(this.opts.extraHeaders);
+
+    if (isPost) {
+      headers.set("content-type", "text/plain;charset=UTF-8");
+    }
+
+    this.cookieJar?.appendCookies(headers);
+
+    return fetch(this.uri(), {
+      method: isPost ? "POST" : "GET",
+      body: isPost ? data : null,
+      headers,
+      credentials: this.opts.withCredentials ? "include" : "omit",
+    }).then((res) => {
+      if (this.cookieJar) {
+        // @ts-ignore getSetCookie() was added in Node.js v19.7.0
+        this.cookieJar.parseCookies(res.headers.getSetCookie());
+      }
+
+      return res;
+    });
+  }
+}