socketio/socket.io · error · Error

Unknown type: ${obj}

Error message

Unknown type: ${obj}

What it means

Thrown by the parser Decoder.add() when add() receives a value that is neither a string, a binary object (ArrayBuffer/Buffer/Blob), nor an object with base64. The else branch at the end of add() catches any input type the decoder does not know how to dispatch.

Source

Thrown at packages/socket.io-parser/lib/index.ts:210

        this.reconstructor = new BinaryReconstructor(packet);
      } else {
        // non-binary full packet
        super.emitReserved("decoded", packet);
      }
    } else if (isBinary(obj) || obj.base64) {
      // raw binary data
      if (!this.reconstructor) {
        throw new Error("got binary data when not reconstructing a packet");
      } else {
        packet = this.reconstructor.takeBinaryData(obj);
        if (packet) {
          // received final buffer
          this.reconstructor = null;
          super.emitReserved("decoded", packet);
        }
      }
    } else {
      throw new Error("Unknown type: " + obj);
    }
  }

  /**
   * Decode a packet String (JSON data)
   *
   * @param {String} str
   * @return {Object} packet
   */
  private decodeString(str): Packet {
    let i = 0;
    // look up type
    const p: any = {
      type: Number(str.charAt(0)),
    };

    if (PacketType[p.type] === undefined) {
      throw new Error("unknown packet type " + p.type);

View on GitHub (pinned to ae7fb46e08)

Solutions

1. Ensure only valid encoded inputs reach add(): an encoded packet string, an ArrayBuffer/Buffer/Blob, or a base64 object.
2. Add an input-type guard at the transport boundary to coerce or reject non-frame values before calling add().
3. If you control the producer, serialize the packet via the Encoder before sending.

Example fix

// before
decoder.add(42); // throws 'Unknown type: 42'

// after
if (typeof value === 'string' || isBinary(value)) {
  decoder.add(value);
} else {
  throw new TypeError('decoder.add expects a string or binary frame');
}

function isDecodable(obj){
  return typeof obj==='string' || isBinary(obj) || (obj && typeof obj==='object' && obj.base64);
}

function isBinary(x){ return x instanceof ArrayBuffer || (typeof Buffer!=='undefined'&&Buffer.isBuffer(x)) || (typeof Blob!=='undefined'&&x instanceof Blob); }

if(!isDecodable(value)) throw new TypeError('decoder.add expects a string or binary frame');
decoder.add(value);

When it happens

Trigger: Calling decoder.add(value) where value is e.g. a number, boolean, plain object (non-base64), null, or undefined.

Common situations: A custom/buggy transport passing non-frame values to the decoder; or a unit test feeding literal numbers/objects instead of encoded packet strings.

Related errors

• invalid payload
• invalid format
• unknown event name: ${eventName}
• got plaintext data when reconstructing a packet
• got binary data when not reconstructing a packet

AI-assisted analysis of socketio/socket.io@ae7fb46e08 (2026-08-03). Data as JSON: /data/errors/f80d4037eed9f7d9.json. Report an issue: GitHub.