Get started with ExpressJS

import type { RequestHandler } from 'express';
import { createGoogleGenerativeAI } from '@ai-sdk/google';
import { streamText } from 'ai';
import type { CoreMessage } from 'ai';
import { verifySignature, streamResponse } from '@layercode/node-server-sdk';
import { Readable } from 'node:stream'; // Node.js 18+ only

const google = createGoogleGenerativeAI({
  apiKey: process.env.GOOGLE_GENERATIVE_AI_API_KEY,
});

const sessionMessages: Record<string, CoreMessage[]> = {};

const SYSTEM_PROMPT =
  "You are a helpful conversation assistant. You should respond to the user's message in a conversational manner. Your output will be spoken by a TTS model. You should respond in a way that is easy for the TTS model to speak and sound natural.";
const WELCOME_MESSAGE = 'Welcome to Layercode. How can I help you today?';

export const onRequestPost: RequestHandler = async (req, res) => {
  const requestBody = req.body;
  const signature = req.header('layercode-signature') || '';
  const secret = process.env.LAYERCODE_WEBHOOK_SECRET || '';
  const payload = JSON.stringify(requestBody);
  const isValid = verifySignature({ payload, signature, secret });
  if (!isValid) {
    console.error('Invalid signature', signature, secret, payload);
    res.status(401).send('Unauthorized');
    return;
  }
  console.log('Request body received from Layercode', requestBody);
  const { session_id, text, type } = requestBody;

  let messages = sessionMessages[session_id] || [];
  messages.push({ role: 'user', content: [{ type: 'text', text }] });

  let response;
  if (type === 'session.start') {
    response = streamResponse(requestBody, async ({ stream }: { stream: any }) => {
      stream.tts(WELCOME_MESSAGE);
      messages.push({
        role: 'assistant',
        content: [{ type: 'text', text: WELCOME_MESSAGE }],
      });
      stream.end();
    });
  } else {
    response = streamResponse(requestBody, async ({ stream }: { stream: any }) => {
      try {
        const { textStream } = streamText({
          model: google('gemini-2.0-flash-001'),
          system: SYSTEM_PROMPT,
          messages,
          onFinish: async ({ response }: { response: any }) => {
            messages.push(...response.messages);
            console.log('Current message history for session', session_id, JSON.stringify(messages, null, 2));
            sessionMessages[session_id] = messages;
          },
        });
        stream.data({
          textToBeShown: 'Hello, how can I help you today?',
        });
        await stream.ttsTextStream(textStream);
      } catch (err) {
        console.error('Handler error:', err);
      } finally {
        console.log('Stream ended');
        stream.end();
      }
    });
  }

  // Set headers and status
  response.headers.forEach((value, key) => res.setHeader(key, value));
  res.status(response.status);

  if (response.body) {
    const nodeStream = Readable.fromWeb(response.body as any);
    nodeStream.pipe(res);
  } else {
    res.end();
  }
};