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Version: 4.x

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

import { Server } from "socket.io";

const io = new Server({
  parser: myParser
});

Client

import { io } from "socket.io-client";

const socket = io({
  parser: myParser
});

Available parsers​

Besides the default parser, here is the list of available parsers:

PackageDescription
socket.io-circular-parserSimilar to the default parser, but handles circular references.
socket.io-msgpack-parserUses MessagePack to encode the packets (based on the notepack.io package).
@skgdev/socket.io-msgpack-javascriptUses MessagePack to encode the packets (based on the @msgpack/msgpack package).
socket.io-json-parserUses JSON.stringify() and JSON.parse() to encode the packets.
socket.io-cbor-x-parserUses cbor-x to encode the packets.
@socket.io/devalue-parserUses devalue to encode the packets.

Implementing your own parser​

Here is a basic example with a parser that uses the JSON.stringify() and JSON.parse() methods:

import { Emitter } from "@socket.io/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)];
  }
}

function isObject(value) {
  return Object.prototype.toString.call(value) === "[object Object]";
}

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 || isObject(data);
      case 1: // DISCONNECT
        return data === undefined;
      case 2: // EVENT
        return Array.isArray(data) && typeof data[0] === "string";
      case 3: // ACK
        return Array.isArray(data);
      case 4: // CONNECT_ERROR
        return isObject(data);
      default:
        return false;
    }
  }
  /**
   * Clean up internal buffers
   */
  destroy() {}
}

export const parser = { 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:

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

import { Server } from "socket.io";
import customParser from "socket.io-msgpack-parser";

const io = new Server({
  parser: customParser
});

Client (Node.js)

import { io } from "socket.io-client";
import customParser from "socket.io-msgpack-parser";

const socket = io("https://example.com", {
  parser: customParser
});

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 result in smaller payloads (especially when using a lot of numbers)

Cons:

info

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.