This commit adds the ability to provide a list of transport
implementations to use when connecting to an Engine.IO server.
This can be used to use HTTP long-polling based on `fetch()`, instead
of the default implementation based on the `XMLHttpRequest` object.
```
import { Socket, Fetch, WebSocket } from "engine.io-client";
const socket = new Socket({
transports: [Fetch, WebSocket]
});
```
This is useful in some environments that do not provide a
`XMLHttpRequest` object, like Chrome extension background scripts.
> XMLHttpRequest() can't be called from a service worker, extension or
otherwise. Replace calls from your background script to
XMLHttpRequest() with calls to global fetch().
Source: https://developer.chrome.com/docs/extensions/develop/migrate/to-service-workers#replace-xmlhttprequest
Related:
- https://github.com/socketio/engine.io-client/issues/716
- https://github.com/socketio/socket.io/issues/4980
This is also useful when running the client with Deno or Bun, as it
allows to use the built-in `fetch()` method and `WebSocket` object,
instead of using the `xmlhttprequest-ssl` and `ws` Node.js packages.
Related: https://github.com/socketio/socket.io-deno/issues/12
This feature also comes with the ability to exclude the code related to
unused transports (a.k.a. "tree-shaking"):
```js
import { SocketWithoutUpgrade, WebSocket } from "engine.io-client";
const socket = new SocketWithoutUpgrade({
transports: [WebSocket]
});
```
In that case, the code related to HTTP long-polling and WebTransport
will be excluded from the final bundle.
Related: https://github.com/socketio/socket.io/discussions/4393
