Updating documents in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express route when updating a document?

Consider this Express route that updates a user document by ID. What will the response be if the user exists and the update is successful?

Express

app.put('/users/:id', async (req, res) => {
  try {
    const updatedUser = await User.findByIdAndUpdate(req.params.id, req.body, { new: true });
    if (!updatedUser) {
      return res.status(404).send('User not found');
    }
    res.json(updatedUser);
  } catch (error) {
    res.status(500).send('Server error');
  }
});

A Responds with the updated user document in JSON format.

B Responds with a 404 status and 'User not found' message regardless of user existence.

C Responds with a 500 status and 'Server error' message on successful update.

D Responds with the original user document before update.

💡 Hint

Look at the option { new: true } in findByIdAndUpdate.

The option { new: true } makes findByIdAndUpdate return the updated document. So the response sends the updated user in JSON.

📝 Syntax

intermediate

Which option correctly updates a document using Mongoose in Express?

You want to update a document by ID and return the updated document. Which code snippet is correct?

A User.update(id, update, { new: true });

B User.findByIdAndUpdate(id, update, { new: true }, callback);

C User.updateOne(id, update, { returnNew: true });

D User.findByIdAndUpdate(id, update); res.send(updatedUser);

💡 Hint

Check the Mongoose method names and options for returning updated documents.

findByIdAndUpdate with { new: true } returns the updated document. updateOne and update do not return the updated document by default.

🔧 Debug

advanced

Why does this Express update route always return the original document instead of the updated one?

Look at this code snippet. Why does it return the original document before update instead of the updated one?

Express

app.put('/items/:id', async (req, res) => {
  const item = await Item.findByIdAndUpdate(req.params.id, req.body);
  res.json(item);
});

A Because res.json cannot send updated documents.

B Because the update data in req.body is ignored by findByIdAndUpdate.

C Because the route method should be POST, not PUT.

D Because findByIdAndUpdate defaults to returning the original document unless { new: true } is set.

💡 Hint

Check the options passed to findByIdAndUpdate.

By default, findByIdAndUpdate returns the document before the update. You must set { new: true } to get the updated document.

❓ state_output

advanced

What is the value of 'result.modifiedCount' after this update operation?

Given this Mongoose update operation, what will be the value of result.modifiedCount if one document was updated successfully?

Express

const result = await User.updateOne({ _id: userId }, { $set: { name: 'Alice' } });
// What is result.modifiedCount?

A 1

B 0

C undefined

D Throws an error

💡 Hint

Check the Mongoose updateOne result object properties.

updateOne returns an object with modifiedCount indicating how many documents were modified. If one document updated, modifiedCount is 1.

🧠 Conceptual

expert

Which option best explains why using findOneAndUpdate with { runValidators: true } is important?

Why should you include { runValidators: true } when updating documents with findOneAndUpdate in Mongoose?

A It automatically creates a new document if none matches the query.

B It returns the original document instead of the updated one.

C It ensures the update respects schema validation rules before saving changes.

D It disables all validation to speed up the update process.

💡 Hint

Think about data integrity and schema rules during updates.

By default, findOneAndUpdate does not run schema validators. Setting runValidators: true enforces validation on the update.