1. Definition

Pre and Post hooks (also called middleware) in Mongoose are functions that run before (pre) or after (post) certain database operations like save, update, delete, etc.

2. Simple Understanding

👉 Pre Hook = runs BEFORE an operation
👉 Post Hook = runs AFTER an operation
👉 Used to add extra logic automatically

3. When to Use

• Before saving data (validation, hashing password)
• After saving data (logging, notifications)
• Before delete/update operations
• When automating repeated logic

4. Where it is Used

• Schema definition
• User authentication systems
• Logging and auditing
• Data processing workflows

5. Why We Use It

• Automates repetitive tasks
• Improves code reusability
• Ensures consistency
• Keeps business logic clean

6. How It Works

• Defined inside schema
• Triggered automatically
• Uses pre() and post() methods
• Runs based on operation type

7. Basic Syntax

// Pre Hook
schema.pre("save", function(next) {
  console.log("Before saving");
  next();
});

// Post Hook
schema.post("save", function(doc) {
  console.log("After saving", doc);
});

8. Real Example

const mongoose = require("mongoose");

const userSchema = new mongoose.Schema({
  name: String,
  password: String
});

// Pre hook (before save)
userSchema.pre("save", function(next) {
  console.log("Hashing password...");
  next();
});

// Post hook (after save)
userSchema.post("save", function(doc) {
  console.log("User saved:", doc.name);
});

const User = mongoose.model("User", userSchema);

9. Explanation

• pre("save") runs before saving document
• post("save") runs after saving document
• next() is required in pre hook
• doc contains saved document in post hook

10. Advantages

• Automates logic
• Cleaner code structure
• Reusable logic
• Centralized control

11. Disadvantages

• Can make debugging harder
• Hidden execution flow
• Performance overhead if overused