Mongoose middleware (pre/post hooks) in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a pre-save hook modifies a document field?

Consider a Mongoose schema with a pre('save') hook that changes a field value before saving. What will be the final value stored in the database?

Express

const userSchema = new mongoose.Schema({ name: String, active: Boolean });
userSchema.pre('save', function(next) {
  this.active = true;
  next();
});
const User = mongoose.model('User', userSchema);

const user = new User({ name: 'Alice', active: false });
await user.save();
console.log(user.active);

A undefined

B true

C false

D null

💡 Hint

The pre-save hook runs before the document is saved to the database and can modify fields.

The pre-save hook sets the active field to true before saving, so the saved document has active: true. The console logs the updated value.

❓ state_output

intermediate

What is the output of a post-remove hook logging the removed document?

Given a Mongoose model with a post('remove') hook that logs the removed document, what will be printed after calling remove() on a document?

Express

const itemSchema = new mongoose.Schema({ name: String });
itemSchema.post('remove', function(doc) {
  console.log(doc.name);
});
const Item = mongoose.model('Item', itemSchema);

const item = new Item({ name: 'Book' });
await item.save();
await item.remove();

A Error: remove is not a function

B undefined

C null

D "Book"

💡 Hint

The post-remove hook receives the removed document as an argument.

The post-remove hook logs the name field of the removed document, which is "Book".

📝 Syntax

advanced

Which option correctly defines a pre-update hook using async/await?

You want to run code before updating a document using updateOne. Which code snippet correctly defines an async pre-update hook?

A schema.pre('updateOne', async function(next) { await someAsyncTask(); next(); });

B schema.pre('updateOne', function(next) { await someAsyncTask(); next(); });

C schema.pre('updateOne', async function() { await someAsyncTask(); });

D schema.pre('updateOne', function() { someAsyncTask().then(() => next()); });

💡 Hint

Async hooks can return a promise or use next callback, but mixing both can cause errors.

Option C correctly uses an async function without next callback. Option C uses await inside a non-async function causing syntax error. Option C mixes async and next callback which is discouraged. Option C calls next without defining it as a parameter causing ReferenceError.

🔧 Debug

advanced

Why does this post-save hook not log the updated document?

A developer writes this post-save hook but it logs the old document state instead of the updated one. Why?

Express

schema.post('save', function(doc) {
  console.log(doc.modifiedField);
});

await Model.findByIdAndUpdate(id, { modifiedField: 'newValue' });

A findByIdAndUpdate does not trigger save middleware

B post-save hook only runs on new documents

C The hook must be pre-save to see changes

D The document passed to post-save is a copy, not updated

💡 Hint

Not all update methods trigger save middleware in Mongoose.

Mongoose's findByIdAndUpdate bypasses save middleware. Only save() triggers pre/post save hooks. To run middleware on updates, use pre/post('findOneAndUpdate') hooks.

🧠 Conceptual

expert

How does Mongoose handle errors thrown in async pre hooks?

If an async pre hook throws an error, what is the effect on the document operation?

A The operation is aborted and the error is passed to the callback or promise rejection

B The error is ignored and the operation continues normally

C The error causes the server to crash immediately

D The hook retries the operation automatically once

💡 Hint

Middleware errors usually stop the operation and propagate the error.

When an async pre hook throws an error, Mongoose aborts the operation and passes the error to the caller, either via callback or promise rejection. This prevents saving or updating invalid data.