socketio/socket.io · error · Error

invalid format

Error message

invalid format

What it means

Thrown by the decode() helper in the basic-websocket-client example when an incoming Socket.IO packet fails validation in isPacketValid(). The decoder parses the type byte and JSON data, then checks the packet shape against the expected per-type contract (CONNECT must carry an object, DISCONNECT must have no data, EVENT must be a non-empty array with a string first element). A failure means the frame did not match any valid Socket.IO packet structure.

Source

Thrown at examples/basic-websocket-client/src/index.js:244

    output += JSON.stringify(packet.data);
  }

  return output;
}

function decode(data) {
  let i = 1; // skip "4" prefix

  const packet = {
    type: parseInt(data.charAt(i++), 10),
  };

  if (data.charAt(i)) {
    packet.data = JSON.parse(data.substring(i));
  }

  if (!isPacketValid(packet)) {
    throw new Error("invalid format");
  }

  return packet;
}

function isPacketValid(packet) {
  switch (packet.type) {
    case SIOPacketType.CONNECT:
      return typeof packet.data === "object";
    case SIOPacketType.DISCONNECT:
      return packet.data === undefined;
    case SIOPacketType.EVENT: {
      const args = packet.data;
      return (
        Array.isArray(args) && args.length > 0 && typeof args[0] === "string"
      );
    }
    default:

View on GitHub (pinned to ae7fb46e08)

Solutions

1. Inspect the raw frame being passed to decode() and confirm its type byte is one of 0 (CONNECT), 1 (DISCONNECT), or 2 (EVENT).
2. If the server legitimately sends other packet types (ACK=3, ERROR=4, BINARY_EVENT=5), extend isPacketValid/SIOPacketType to model them.
3. Wrap the decode() call in try/catch and skip/log frames that are not understood rather than crashing the client.

Example fix

// before
const packet = decode(raw);
handle(packet);

// after
let packet;
try {
  packet = decode(raw);
} catch (e) {
  console.warn('dropping malformed frame', raw, e.message);
  return;
}
handle(packet);

function safeType(t){ return [0,1,2].includes(t); }
function looksValid(raw){
  const t = parseInt(raw.charAt(1),10);
  if(!safeType(t)) return false;
  // rough shape checks matching isPacketValid
  return true;
}

function isPacketShape(p){
  switch(p.type){
    case 0: return typeof p.data === 'object' && p.data !== null;
    case 1: return p.data === undefined;
    case 2: return Array.isArray(p.data) && p.data.length>0 && typeof p.data[0]==='string';
    default: return false;
  }
}

let packet;
try { packet = decode(raw); }
catch (e) { console.warn('dropping frame', raw, e.message); return; }
handle(packet);

When it happens

Trigger: Calling decode() on a raw string that is malformed: a CONNECT (type 0) whose JSON body is not an object, a DISCONNECT (type 1) that carries trailing data, or an EVENT (type 2) whose payload is missing or whose first array element is not a string. Any type byte outside 0/1/2 also fails because isPacketValid returns false in the default branch.

Common situations: Manually decoding frames with a custom/naive parser in the example client, or pointing the example client at a server/endpoint that sends a non-standard or Engine.IO-only frame (e.g. an ERROR packet type 4 which this minimal client does not model).

Related errors

• unknown event name: ${eventName}
• Unknown type: ${obj}
• invalid payload
• illegal attachments
• got plaintext data when reconstructing a packet

AI-assisted analysis of socketio/socket.io@ae7fb46e08 (2026-08-03). Data as JSON: /data/errors/dcebbadd3f8bcf40.json. Report an issue: GitHub.