What is Mongoose middleware (hooks)? | MongoDB Interview Questions

What You'll Learn

Pre middleware
Post middleware
Common use cases

What is Middleware?

Middleware (hooks) are functions executed before or after certain operations like save, validate, remove.

Pre Middleware

code.jsJavaScript

// Hash password before saving
userSchema.pre('save', async function(next) {
  if (!this.isModified('password')) return next();
  this.password = await bcrypt.hash(this.password, 10);
  next();
});

// Update timestamp
userSchema.pre('save', function(next) {
  this.updatedAt = new Date();
  next();
});

// Pre find - exclude deleted
userSchema.pre(/^find/, function(next) {
  this.where({ deleted: { $ne: true } });
  next();
});

Post Middleware

code.jsJavaScript

// After save
userSchema.post('save', function(doc) {
  console.log('User saved:', doc._id);
});

// After remove
userSchema.post('remove', async function(doc) {
  await Comment.deleteMany({ userId: doc._id });
});

// Error handling
userSchema.post('save', function(error, doc, next) {
  if (error.code === 11000) {
    next(new Error('Email already exists'));
  } else {
    next(error);
  }
});

Query Middleware

code.jsJavaScript

// Pre findOneAndUpdate
userSchema.pre('findOneAndUpdate', function(next) {
  this.set({ updatedAt: new Date() });
  next();
});

Aggregate Middleware

code.jsJavaScript

userSchema.pre('aggregate', function(next) {
  this.pipeline().unshift({ $match: { active: true } });
  next();
});