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[Part-1] Text to Action: Build a Smart Calendar AI Assistant

[Part-1] Text to Action: Build a Smart Calendar AI Assistant

Integrate Google Calendar into your app by setting up OAuth2 authentication and creating an Express endpoint to add events, all in under 50 lines of code.

👁 Vivek Vellaiyappan Surulimuthu user avatar

Vivek Vellaiyappan Surulimuthu

Mar. 20, 25 · Tutorial

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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.

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.

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

JavaScript

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

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

Project Structure

JavaScript

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

The Code: Complete Express Application

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('/');
});

// 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:

Plain Text

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:

JavaScript

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:

JavaScript

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

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:

JavaScript

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

OAuth2 Authentication Flow

User visits /auth/google endpoint.
User is redirected to the Google consent screen.
After granting access, Google redirects to /auth/google/callback.
The app stores OAuth tokens for future API calls.
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.

API Express Google Calendar

Published at DZone with permission of Vivek Vellaiyappan Surulimuthu. See the original article here.

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URL: https://dzone.com/articles/build-a-google-calendar-api-with-expressjs