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Overview

The alert system automatically notifies all registered users when a tracked product’s price drops. Alerts are sent via Telegram with rich formatting, product images, and direct links to Amazon.

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When Alerts Trigger

Alerts are triggered when the current scraped price is lower than the previously stored price: 
// Detect price drop compared to stored.price (originalPrice)
if (scraped.price < originalPrice) {
  const diff = (originalPrice - scraped.price).toFixed(2);
  const pct = (((originalPrice - scraped.price) / originalPrice) * 100).toFixed(1);

  const msg = `🚨 ¡Precio reducido!

*${escapeMD(updated.title)}*

💰 Precio anterior: $${originalPrice}
🎯 Precio actual: $${scraped.price}
💵 Ahorro: $${diff} (${pct}% menos)
📉 Histórico más bajo: $${updated.lowestPrice}

[Ver en Amazon](${sanitized})`;

  productsChanged.push({ url: sanitized, message: msg, imageUrl: updated.imageUrl });
}
Only price drops trigger alerts. Price increases are tracked but don’t generate notifications.

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Alert Message Format

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With Product Image

When the product has an image URL, alerts are sent as photo messages: 
if (p.imageUrl) {
  await bot.sendPhoto(chatId, p.imageUrl, {
    caption: p.message,
    parse_mode: "Markdown",
    reply_markup: { 
      inline_keyboard: [[{ text: "🛒 Ver en Amazon", url: p.url }]] 
    }
  });
}

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Example Alert Message

🚨 ¡Precio reducido!

*Apple AirPods Pro (2nd Generation)*

💰 Precio anterior: $249.99
🎯 Precio actual: $199.99
💵 Ahorro: $50.00 (20.0% menos)
📉 Histórico más bajo: $189.99

[Ver en Amazon](https://www.amazon.com/dp/B0CHWRXH8B)

Previous Price

Shows the last known price before the drop

Current Price

Displays the new, lower price

Savings

Calculates both dollar amount and percentage saved

Historical Low

Shows the lowest price ever recorded

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Multi-Chat Broadcasting

When a price drop is detected, the alert is broadcast to all registered chats: 
// Notify chats about changes
if (productsChanged.length) {
  console.log(`🎉 ${productsChanged.length} productos cambiados — notificando a ${chats.size} chats.`);
  
  for (const chatId of chats) {
    for (const p of productsChanged) {
      try {
        if (p.imageUrl) {
          await bot.sendPhoto(chatId, p.imageUrl, {
            caption: p.message,
            parse_mode: "Markdown",
            reply_markup: { inline_keyboard: [[{ text: "🛒 Ver en Amazon", url: p.url }]] }
          });
        } else {
          await bot.sendMessage(chatId, p.message, {
            parse_mode: "Markdown",
            reply_markup: { inline_keyboard: [[{ text: "🛒 Ver en Amazon", url: p.url }]] }
          });
        }
      } catch (err) {
        console.error(`❌ Error enviando notificación a ${chatId}:`, err.message);
      }
      // small pause between messages
      await new Promise((r) => setTimeout(r, 500));
    }
  }
} else {
  console.log("ℹ️ No se detectaron bajadas de precio en esta pasada.");
}
A 500ms delay is added between each notification to avoid rate limiting by Telegram’s API.

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Chat Registration

Users are automatically registered when they:
  1. Send the /start command
  2. Send any message to the bot
// Auto-register chats on any message (only once)
bot.on("message", (msg) => {
  const chatId = msg.chat.id;
  if (!chats.has(chatId)) {
    chats.add(chatId);
    saveData();
    console.log(`🎉 Nuevo chat registrado (por mensaje): ${chatId}`);
  }
});
Registered chats are persisted in prices.json: 
{
  "products": { ... },
  "chats": [123456789, 987654321]
}

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Markdown Escaping

To ensure proper Telegram formatting, special characters are escaped: 
// Escape for Markdown (basic). This function escapes characters that break Markdown.
function escapeMD(text = "") {
  return String(text).replace(/([_*[\]()~`>#+\-=|{}.!])/g, "\\$1");
}
This prevents product titles with special characters from breaking the message formatting.

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Inline Buttons

Each alert includes an inline button for quick access to the product: 
reply_markup: { 
  inline_keyboard: [[{ text: "🛒 Ver en Amazon", url: p.url }]] 
}
Clicking the button opens the product page directly in the user’s browser.

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Alert Frequency

Alerts are sent in two scenarios:
  1. Automated checks: Every 2 hours via cron job
  2. Manual checks: When a user runs /check command
// Check prices every 2 hours
cron.schedule("0 */2 * * *", async () => {
  console.log("🔄 Cron: revisión automática de precios...");
  try {
    await checkPrices();
  } catch (err) {
    console.error("❌ Error en cron checkPrices:", err.message);
  }
});
The checkPrices() function automatically sends alerts when price drops are detected.

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Error Handling

If sending an alert fails, the error is logged but doesn’t stop other notifications: 
try {
  await bot.sendPhoto(chatId, p.imageUrl, { ... });
} catch (err) {
  console.error(`❌ Error enviando notificación a ${chatId}:`, err.message);
}
Failed notifications to one chat don’t affect alerts to other registered users.

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No Duplicate Alerts

The system prevents duplicate alerts by only triggering when scraped.price < originalPrice. Once an alert is sent, the new lower price becomes the comparison baseline for future checks.

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Example Flow

  1. Product price: $100 (stored)
  2. Next check finds: $80 → Alert sent ✅
  3. New stored price: $80
  4. Next check finds: $80 → No alert (price unchanged)
  5. Next check finds: $75 → Alert sent ✅

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Testing Alerts

Use the /check command to immediately test the alert system: 
/check
Response: 
⏳ Revisando precios de todos los productos... (esto puede tardar)
✅ Revisión completada. Si hubo cambios, los notifiqué.
If price drops are detected during the check, alerts will be sent automatically.

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Fallback Without Images

Some products may not have images. In this case, alerts are sent as text-only messages: 
} else {
  await bot.sendMessage(chatId, p.message, {
    parse_mode: "Markdown",
    reply_markup: { inline_keyboard: [[{ text: "🛒 Ver en Amazon", url: p.url }]] }
  });
}
The message format remains the same, just without the visual element.

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