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date: 2025-07-29 topics: [nodejs, backend, likes-dislikes, mern, system-design] day: 13

📘 Day 13 – Like/Dislike System Like Twitter 💥

✅ What I Worked On

Built a fully working Like/Dislike system in DevFlow — inspired by how platforms like Twitter or Reddit manage user reactions in real time.

Feature	Status
👍 Like a post	✅ Done
👎 Dislike a post	✅ Done
🔁 Toggle reactions cleanly	✅ Handled
🧠 Distinguish between Question and Answer	✅ Via targetType
🔐 Protected endpoints with auth	✅ Secured

🧠 Design Decisions

🔄 Unified Reaction Schema

{
  _id: ObjectId,
  userId: ObjectId,
  targetId: ObjectId,       // Question or Answer ID
  targetType: "question" | "answer",
  reaction: "like" | "dislike",
  createdAt: Date
}

✅ Single schema handles both Questions and Answers ✅ Easily filter and aggregate by type or target

♻️ Reaction Toggle Logic

• If user likes a post already, remove like (toggle off)
• If switching from like → dislike or vice versa → update reaction
• Ensures one reaction per user per post

🧱 Scalable Design with Mongoose + Aggregation

• Used $match + $group to count reactions efficiently
• Indexes added on targetId and userId for fast toggles & counts

🔒 Protected Endpoints

• Only authenticated users can react
• Validated targetId existence before saving

📂 Project Structure

• ✅ Modular service functions (clean toggle logic)
• ✅ Reusable controller for all types (question, answer)
• ✅ Express REST APIs follow standard resource naming

📸 Screenshot

❌ Blockers

• Had to handle edge cases: user re-likes the same post, etc.
• Early version had duplicate reactions before applying $set logic

🧠 Reflection

Building reactions wasn’t just about toggling a field. It taught me how apps scale this data for:

• Real-time counts
• Feed personalization
• Behavior analytics

“Big features start with small schemas — but scale with smart design.”

🔜 Next Up

• Add View Aggregation: track impressions per post
• Build User Reaction History page
• Cache reaction counts (optional)

🔗 References / Code

• 💻 DevFlow GitHub Repo
• 💻 DevFlow commit