Build a Smart Calendar AI Assistant - The Legend of Hanuman
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Build a Smart Calendar AI Assistant

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Welcome to the “Text to Action” series, where we build intelligent systems that transform natural language into real-world actionable outcomes using AI.

To understand the concept better, let’s start simple by building a Smart Calendar AI Assistant. Soon, we’ll tackle more complex challenges — from smart home control to document automation — as we master the art of turning words into actions.

https://www.youtube.com/watch?v=watch

Our goal for this project is to create a working AI system where you can simply type or say, “Create a party event at 5 pm on March 20” and watch it instantly appear in your Google Calendar.

Part 1 focuses on building the backend foundation: an Express.js backend that connects to Google Calendar’s API. This will handle the actual event creation before we add natural language processing in future episodes.

Let’s begin by creating our calendar integration!

This tutorial shows you how to authenticate with OAuth2 and create a simple Express endpoint that adds events to your Google Calendar — perfect for integrating calendar functionality into any application.

Create a simple REST API endpoint that adds events to Google Calendar with minimal code.

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What We’re Building

A lightweight Express.js API that exposes a single endpoint for creating Google Calendar events. This API will:

  • Authenticate with Google using OAuth2
  • Add events to your primary calendar
  • Return event details and links

Prerequisites

# Install Node.js (v14+) and npm
npm install express googleapis dotenv

You’ll need OAuth credentials from the Google Cloud Console.

Project Structure

text-to-calendar/
├── app.js         # Our entire application
├── public/        # Static files
│   └── index.html # Simple web interface
└── .env           # Environment variables

The Code: Complete Express Application

{
try {
// Check if we have the required fields
const
error: error.message = req.body;

if (!summary || !startDateTime || !endDateTime) {
return res.status(400).json( ‘Failed to create event’
);
}

// Create the calendar event
const calendar = google.calendar({ version: ‘v3’, auth: oauth2Client });
const response = await calendar.events.insert({
calendarId: ‘primary’,
resource: {
summary,
description: description || summary,
start: { dateTime: startDateTime },
end: { dateTime: endDateTime }
}
});

res.status(201).json({
success: true,
eventId: response.data.id,
eventLink: response.data.htmlLink
});
} catch (error) {
console.error(‘Error creating event:’, error);
res.status(error.code || 500).json({
error: error.message || ‘Failed to create event’
});
}
});

// Start server
const PORT = process.env.PORT || 3000;
app.listen(PORT, () => console.log(`Server running at http://localhost:${PORT}`));” data-lang=”text/javascript”>

// app.js - Google Calendar API with Express
require('dotenv').config();
const express = require('express');
const { google } = require('googleapis');
const fs = require('fs');
const path = require('path');

const app = express();
app.use(express.json());
app.use(express.static('public'));

// Configure OAuth
const oauth2Client = new google.auth.OAuth2(
  process.env.GOOGLE_CLIENT_ID,
  process.env.GOOGLE_CLIENT_SECRET,
  process.env.GOOGLE_REDIRECT_URI || 'http://localhost:3000/auth/google/callback'
);

// Load saved tokens if available
try {
  const tokens = JSON.parse(fs.readFileSync('tokens.json'));
  oauth2Client.setCredentials(tokens);
} catch (e) { /* No tokens yet */ }

// Auth routes
app.get('/auth/google', (req, res) => {
  const authUrl = oauth2Client.generateAuthUrl({
    access_type: 'offline',
    scope: ['https://www.googleapis.com/auth/calendar']
  });
  res.redirect(authUrl);
});

app.get('/auth/google/callback', async (req, res) => {
  const { tokens } = await oauth2Client.getToken(req.query.code);
  oauth2Client.setCredentials(tokens);
  fs.writeFileSync('tokens.json', JSON.stringify(tokens));
  res.redirect("https://feeds.dzone.com/");
});

// API endpoint to create calendar event
app.post('/api/create-event', async (req, res) => {
  try {
    // Check if we have the required fields
    const { summary, description, startDateTime, endDateTime } = req.body;
    
    if (!summary || !startDateTime || !endDateTime) {
      return res.status(400).json({ 
        error: 'Missing required fields: summary, startDateTime, endDateTime' 
      });
    }
    
    // Create the calendar event
    const calendar = google.calendar({ version: 'v3', auth: oauth2Client });
    const response = await calendar.events.insert({
      calendarId: 'primary',
      resource: {
        summary,
        description: description || summary,
        start: { dateTime: startDateTime },
        end: { dateTime: endDateTime }
      }
    });
    
    res.status(201).json({
      success: true,
      eventId: response.data.id,
      eventLink: response.data.htmlLink
    });
  } catch (error) {
    console.error('Error creating event:', error);
    res.status(error.code || 500).json({ 
      error: error.message || 'Failed to create event' 
    });
  }
});

// Start server
const PORT = process.env.PORT || 3000;
app.listen(PORT, () => console.log(`Server running at http://localhost:${PORT}`));

How to Use

1. Set up Environment Variables

Create a .env file:

GOOGLE_CLIENT_ID=your_client_id
GOOGLE_CLIENT_SECRET=your_client_secret
GOOGLE_REDIRECT_URI=http://localhost:3000/auth/google/callback
PORT=3000

2. Authenticate With Google

Visit http://localhost:3000/auth/google in your browser to connect to Google Calendar.

3. Create an Event

Use a POST request to the API endpoint:

curl -X POST http://localhost:3000/api/create-event \
  -H "Content-Type: application/json" \
  -d '{
    "summary": "Team Meeting",
    "description": "Weekly team status update",
    "startDateTime": "2025-03-10T14:00:00-07:00",
    "endDateTime": "2025-03-10T15:00:00-07:00"
  }'

Sample response:

{
  "success": true,
  "eventId": "e0ae1vv8gkop6bcbb5gqilotrs",
  "eventLink": "https://www.google.com/calendar/event?eid=ZTBhZTF2djhna29wNmJjYmI1Z3FpbG90cnMgdml2ZWtzbWFpQG0"
}

An event is created

API Endpoint Details

POST /api/create-event – Create a calendar event

Request:

  • summary (String, required) – Event title
  • description (String) – Event details
  • startDateTime (ISO 8601) – When event starts
  • endDateTime (ISO 8601) – When event ends

Response format:

{
  "success": true,
  "eventId": "e0ae1vv8gkop6bcbb5gqilotrs",
  "eventLink": "https://www.google.com/calendar/event?eid=..."
}

OAuth2 Authentication Flow

  1. User visits /auth/google endpoint.
  2. User is redirected to the Google consent screen.
  3. After granting access, Google redirects to /auth/google/callback. 
  4. The app stores OAuth tokens for future API calls.
  5. API is now ready to create events.

Troubleshooting

OAuth setup can be tricky. If you encounter issues, refer to the OAUTH_SETUP.md in the repository, which contains a detailed troubleshooting guide for common OAuth errors.

Security Considerations

  • Store OAuth tokens securely in production (not in a local file)
  • Use HTTPS for all API endpoints in production
  • Consider rate limiting to prevent abuse
  • Implement proper error handling and validation

Conclusion

With just under 60 lines of core code, we’ve created a functional API that connects to Google Calendar. This foundation can be extended to support more calendar operations like retrieving, updating, or deleting events.

The complete code is available on text-to-calendar-ai.

The same approach can be applied to integrate with other Google services or third-party APIs that use OAuth2 authentication.


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