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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:

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.