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Overview

The Price Tracker Bot requires minimal configuration to get started. This guide walks you through creating the necessary configuration files, setting up data persistence, and ensuring proper file permissions.

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Prerequisites

Before setting up the bot, ensure you have:
  • Node.js 16+ installed
  • A Telegram Bot Token from @BotFather
  • Playwright browsers installed (npx playwright install chromium)
The bot uses Playwright with Chromium for web scraping. The first run may take longer while Playwright downloads the browser binaries.

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Environment File Setup

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Creating the .env File

The bot uses a .env file to store sensitive configuration. Create this file in your project root: 
touch .env
Add your Telegram bot token:
.env
TELEGRAM_TOKEN=your_bot_token_here
Never commit your .env file to version control! Add it to your .gitignore:
echo ".env" >> .gitignore

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Token Validation

The bot validates the presence of TELEGRAM_TOKEN at startup. If the token is missing, the application will exit immediately:
index.mjs (lines 11-15)
const TELEGRAM_TOKEN = process.env.TELEGRAM_TOKEN;
if (!TELEGRAM_TOKEN) {
  console.error("❌ TELEGRAM_TOKEN no definido en .env");
  process.exit(1);
}
This validation ensures the bot never runs without proper authentication credentials.

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Data Persistence Configuration

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Storage File

The bot stores all product data and chat subscriptions in a JSON file:
index.mjs (line 20)
const DATA_FILE = "prices.json";
This file is automatically created on first save and contains:
  • products: All tracked products with price history
  • chats: Registered chat IDs for notifications
The prices.json file is created automatically in the same directory as your bot script. No manual creation is needed.

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Data Structure

The persistence file follows this structure: 
{
  "products": {
    "https://amazon.com/dp/PRODUCT123": {
      "url": "https://amazon.com/dp/PRODUCT123",
      "title": "Example Product",
      "price": 29.99,
      "lowestPrice": 24.99,
      "imageUrl": "https://...",
      "addedDate": "2026-03-10T12:00:00.000Z",
      "lastChecked": "2026-03-10T14:30:00.000Z",
      "addedBy": 123456789,
      "history": [
        { "date": "2026-03-10T12:00:00.000Z", "price": 29.99 },
        { "date": "2026-03-10T14:00:00.000Z", "price": 27.99 }
      ]
    }
  },
  "chats": [123456789, 987654321]
}

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Loading Data on Startup

The bot loads existing data during initialization:
index.mjs (lines 24-39)
function loadData() {
  if (fs.existsSync(DATA_FILE)) {
    try {
      const saved = JSON.parse(fs.readFileSync(DATA_FILE, "utf8"));
      priceData = saved.products || {};
      chats = new Set(saved.chats || []);
      console.log(`✅ Datos cargados. Productos: ${Object.keys(priceData).length}, Chats: ${chats.size}`);
    } catch (err) {
      console.error("❌ Error al cargar datos:", err.message);
      priceData = {};
      chats = new Set();
    }
  } else {
    console.log("ℹ️ No existe prices.json — se creará al guardar.");
  }
}
If the file doesn’t exist or is corrupted, the bot starts with empty data structures.

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History Limit Configuration

To prevent unbounded growth of price history, the bot limits stored records:
index.mjs (line 69)
const HISTORY_LIMIT = 120;
With price checks every 2 hours, this provides approximately 10 days of history per product.
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HISTORY_LIMIT
number
default:"120"
Maximum number of price history entries to retain per product. Older entries are automatically removed.
You can modify HISTORY_LIMIT directly in the source code. Higher values provide longer history but increase file size.

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File Permissions

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Read/Write Access

Ensure the bot has proper permissions to create and modify prices.json: 
# Check current permissions
ls -la prices.json

# Set read/write permissions for the user
chmod 600 prices.json
The bot needs write access to its working directory. Running in read-only filesystems will cause data loss.

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Directory Permissions

The bot should run in a directory where it can create files: 
# Verify directory is writable
test -w . && echo "Directory is writable" || echo "No write access"

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Running the Bot

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First Launch

After configuration, start the bot: 
node index.mjs
Successful startup shows: 
✅ Datos cargados. Productos: 0, Chats: 0
🚀 Bot iniciado. Esperando comandos...

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Testing Configuration

  1. Send /start to your bot in Telegram
  2. Add a test product with /add [amazon-url]
  3. Verify data persistence by restarting the bot and checking the product is still tracked
Use /stats to verify your configuration is working correctly. It should show your tracked products and registered chats.

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Backup Strategy

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Manual Backups

Regularly backup your prices.json file: 
# Create timestamped backup
cp prices.json "prices.backup.$(date +%Y%m%d_%H%M%S).json"

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Automated Backups

Consider using a cron job for automatic backups: 
# Backup daily at 3 AM
0 3 * * * cp /path/to/prices.json /backups/prices.$(date +\%Y\%m\%d).json
The bot automatically saves data after every change, so you don’t need to manually trigger saves.

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Troubleshooting

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Bot Won’t Start

Symptom: Bot exits immediately with “TELEGRAM_TOKEN no definido” Solution:
  1. Verify .env file exists in the project root
  2. Check the token is on a line starting with TELEGRAM_TOKEN=
  3. Ensure no spaces around the = sign

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Data Not Persisting

Symptom: Products disappear after restart Solution:
  1. Check file permissions on prices.json
  2. Verify disk space is available: df -h
  3. Look for write errors in bot console output

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Corrupted Data File

Symptom: Bot starts with empty data despite existing prices.json Solution:
  1. Validate JSON syntax: jq . prices.json
  2. Restore from backup if available
  3. Bot will create fresh file if current one is deleted

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Next Steps

Environment Variables

Learn about all available configuration options

Scheduling

Configure automated price checks and summaries

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