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

The source code of this parser can be found here: https://github.com/darrachequesne/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:

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