Custom parser
Since Socket.IO v2.0.0, it is now possible to provide your own parser, in order to control the marshalling / unmarshalling of packets.
Server
const httpServer = require("http").createServer();
const io = require("socket.io")(httpServer, {
parser: myParser
});
Client
const socket = io({
parser: myParser
});
Implementing your own parser​
Here is a basic example with a parser that uses the JSON.stringify()
and JSON.parse()
methods:
const Emitter = require("component-emitter"); // polyfill of Node.js EventEmitter in the browser
class Encoder {
/**
* Encode a packet into a list of strings/buffers
*/
encode(packet) {
return [JSON.stringify(packet)];
}
}
class Decoder extends Emitter {
/**
* Receive a chunk (string or buffer) and optionally emit a "decoded" event with the reconstructed packet
*/
add(chunk) {
const packet = JSON.parse(chunk);
if (this.isPacketValid(packet)) {
this.emit("decoded", packet);
} else {
throw new Error("invalid format");
}
}
isPacketValid({ type, data, nsp, id }) {
const isNamespaceValid = typeof nsp === "string";
const isAckIdValid = id === undefined || Number.isInteger(id);
if (!isNamespaceValid || !isAckIdValid) {
return false;
}
switch (type) {
case 0: // CONNECT
return data === undefined || typeof data === "object";
case 1: // DISCONNECT
return data === undefined;
case 2: // EVENT
return Array.isArray(data) && data.length > 0;
case 3: // ACK
return Array.isArray(data);
case 4: // CONNECT_ERROR
return typeof data === "object";
default:
return false;
}
}
/**
* Clean up internal buffers
*/
destroy() {}
}
module.exports = { Encoder, Decoder };
The default parser​
The source code of the default parser (the socket.io-parser
package) can be found here: https://github.com/socketio/socket.io-parser
Example of output:
- basic emit
socket.emit("test", 42);
will be encoded as:
2["test",42]
||
|└─ JSON-encoded payload
└─ packet type (2 => EVENT)
- emit with binary, acknowledgement and custom namespace
socket.emit("test", Uint8Array.from([42]), () => {
console.log("ack received");
});
will be encoded as:
51-/admin,13["test",{"_placeholder":true,"num":0}]
|||| || └─ JSON-encoded payload with placeholders for binary attachments
|||| |└─ acknowledgement id
|||| └─ separator
|||└─ namespace (not included when it's the main namespace)
||└─ separator
|└─ number of binary attachments
└─ packet type (5 => BINARY EVENT)
and an additional attachment (the extracted Uint8Array)
Pros:
- the binary attachments is then base64-encoded, so this parser is compatible with browsers that do not support Arraybuffers, like IE9
Cons:
- packets with binary content are sent as two distinct WebSocket frames (if the WebSocket connection is established)
The msgpack parser​
This parser uses the MessagePack serialization format.
The source code of this parser can be found here: https://github.com/socketio/socket.io-msgpack-parser
Sample usage:
Server
const httpServer = require("http").createServer();
const io = require("socket.io")(httpServer, {
parser: require("socket.io-msgpack-parser")
});
Client (Node.js)
const socket = require("socket.io-client")("https://example.com", {
parser: require("socket.io-msgpack-parser")
});
In the browser, there is now an official bundle which includes this parser:
- https://cdn.socket.io/3.1.3/socket.io.msgpack.min.js
- cdnjs: https://cdnjs.cloudflare.com/ajax/libs/socket.io/3.1.3/socket.io.msgpack.min.js
- jsDelivr: https://cdn.jsdelivr.net/npm/socket.io-client@3.1.3/dist/socket.io.msgpack.min.js
- unpkg: https://unpkg.com/socket.io-client@3.1.3/dist/socket.io.msgpack.min.js
In that case, you don't need to specify the parser
option.
Pros:
- packets with binary content are sent as one single WebSocket frame (if the WebSocket connection is established)
- may results in smaller payloads (especially when using a lot of numbers)
Cons:
- incompatible with browsers that do not support Arraybuffers, like IE9
- harder to debug in the Network tab of the browser
Please note that socket.io-msgpack-parser
relies on the notepack.io
MessagePack implementation. This implementation mainly focuses on performance and minimal bundle size, and thus does not support features like extension types. For a parser based on the official JavaScript implementation, please check this package.