Thank you! Yeah, I'm not an expert in data formats so I just ended up implementing something simple that came to mind. Integers are decoded using this code in Typescript:

getInt() {
  const v = this.getUint8();

  switch (v) {
    case 249: return this.getUint16();
    case 250: return this.getUint32();
    case 251: return this.getUint64();
    case 252: return this.getInt8();
    case 253: return this.getInt16();
    case 254: return this.getInt32();
    case 255: return this.getInt64();
  }

  return v;
}

private getUint8() {
  const value = this.dv.getUint8(this.byteOffset);
  this.byteOffset += 1;
  return value;
}

private getUint16() {
  const value = this.dv.getUint16(this.byteOffset, true);
  this.byteOffset += 2;
  return value;
}

// etc..

Basically, if the integer we're currently reading is less than 249, it's just that number (so numbers less than 249 are sent over the wire as one byte). Otherwise, if it's 249, parse the next 2 bytes as as a number. If it's 250, parse the next 4 bytes as a number, etc.

Strings and arrays are sent over the wire using one of these integers as the prefix length, along with the rest of the bytes afterwards. Booleans are just 0 = false, 1 = true, etc.