Subclassing Web Standard TransformStream for Encrypted Server-Sent Streaming

export class EncryptedTextTransformer extends TransformStream {
  #secretKey;
 
  constructor(secretKey) {
    if (typeof secretKey !== 'string' || secretKey.length === 0) {
      throw new Error('Encryption key must be a non-empty string');
    }
 
    let key = secretKey;
    super({
      transform(chunk, controller) {
        try {
          const encrypted = Array.from(String(chunk))
            .map((char, i) => String.fromCharCode(char.charCodeAt(0) ^ key.charCodeAt(i % key.length)))
            .join('');
          controller.enqueue(Buffer.from(encrypted).toString('base64') + '\n');
        } catch (err) {
          controller.error(new Error(`Encryption transform failed: ${err.message}`));
        }
      }
    });
    this.#secretKey = secretKey;
  }
}