In Socket.IO v2, the Socket query option was appended to the namespace
in the CONNECT packet:
{
type: 0,
nsp: "/my-namespace?abc=123"
}
Note: the "query" option on the client-side (v2) will be found in the
"auth" attribute on the server-side:
```
// client-side
const socket = io("/nsp1", {
query: {
abc: 123
}
});
socket.query = { abc: 456 };
// server-side
const io = require("socket.io")(httpServer, {
allowEIO3: true // enable compatibility mode
});
io.of("/nsp1").on("connection", (socket) => {
console.log(socket.handshake.auth); // { abc: 456 } (the Socket query)
console.log(socket.handshake.query.abc); // 123 (the Manager query)
});
More information here: https://socket.io/docs/v3/migrating-from-2-x-to-3-0/#Add-a-clear-distinction-between-the-Manager-query-option-and-the-Socket-query-option
Related: https://github.com/socketio/socket.io/issues/3791
When handling compression at the proxy server level, the client receives a weak ETag.
Weak ETags are prefixed with `W/`, e.g. `W/"2.2.0"`.
Upon cache validation we should take care of these too.
Reference: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag
In order to ease the migration to Socket.IO v3, the Socket.IO server
can now communicate with v2 clients.
```js
const io = require("socket.io")({
allowEIO3: true
});
```
This feature is disabled by default.
This functionality was removed in [1] (included in 3.0.0), but
catch-all listeners and socket middleware features are complementary
rather than mutually exclusive.
The only difference with the previous implementation is that passing an
error to the `next` handler will create an error on the server-side,
and not on the client-side anymore.
```js
io.on("connection", (socket) => {
socket.use(([ event, ...args ], next) => {
next(new Error("stop"));
});
socket.on("error", (err) => {
// to restore the previous behavior
socket.emit("error", err);
// or close the connection, depending on your use case
socket.disconnect(true);
});
});
```
This creates additional possibilities about custom error handlers, which
may be implemented in the future.
```js
// user-defined error handler
socket.use((err, [ event ], next) => {
// either handle it
socket.disconnect();
// or forward the error to the default error handler
next(err);
});
// default error handler
socket.use((err, _, next) => {
socket.emit("error", err);
});
```
Related: https://github.com/socketio/socket.io/issues/3678
[1]: 5c73733985
So that the following example:
```js
const io = require('socket.io')({
pingTimeout: 10000
});
io.listen(3000);
```
behaves the same as:
```js
const io = require('socket.io')(3000, {
pingTimeout: 10000
});
```
Before this change, the options in the first example were not forwarded
to the Engine.IO constructor, which is not really intuitive.
The previous syntax (which is still valid):
```js
const io = require('socket.io')();
io.listen(3000, {
pingTimeout: 10000
});
```
