Finding and querying documents in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express route when querying documents?

Consider this Express route that queries a MongoDB collection for users with age greater than 25:

app.get('/users', async (req, res) => {
  const users = await User.find({ age: { $gt: 25 } });
  res.json(users);
});

What will the response contain?

Express

app.get('/users', async (req, res) => {
  const users = await User.find({ age: { $gt: 25 } });
  res.json(users);
});

A A JSON array of all user documents regardless of age

B A JSON array of user documents where each user's age is greater than 25

C A JSON object with a single user document where age equals 25

D An error because $gt is not a valid query operator

💡 Hint

Think about what the MongoDB $gt operator does in a query.

The $gt operator selects documents where the value of the field is greater than the specified value. Here, it returns all users with age > 25 as an array.

📝 Syntax

intermediate

Which option correctly queries documents with multiple conditions in Express?

You want to find all products with price less than 100 and category 'books'. Which query is correct?

Express

const products = await Product.find( /* your query here */ );

A { price: { $lt: 100 }, category: 'books' }

B { price: { $lt: 100 } && category: 'books' }

C { price: { $lt: 100 }, category == 'books' }

D { price: { $lt: 100 }, category: ['books'] }

💡 Hint

Remember how to write multiple conditions in a MongoDB query object.

MongoDB queries use an object with key-value pairs for each condition. The correct syntax is { price: { $lt: 100 }, category: 'books' }.

🔧 Debug

advanced

Why does this Express query return an empty array unexpectedly?

Given this code:

app.get('/search', async (req, res) => {
  const term = req.query.term;
  const results = await Item.find({ name: { $regex: term } });
  res.json(results);
});

Users report no results even when matching items exist. What is the likely cause?

Express

app.get('/search', async (req, res) => {
  const term = req.query.term;
  const results = await Item.find({ name: { $regex: term } });
  res.json(results);
});

A The query should use $text instead of $regex for string matching

B The $regex operator is invalid in MongoDB queries

C The regex is case-sensitive by default, so matches may fail if case differs

D The term variable is undefined because req.query.term is missing

💡 Hint

Think about how MongoDB regex queries handle letter case.

MongoDB regex queries are case-sensitive by default. If the search term case does not match the stored data, no results appear. Adding the 'i' option makes it case-insensitive.

❓ state_output

advanced

What is the value of 'count' after this Express route runs?

Consider this route that counts documents matching a condition:

app.get('/count', async (req, res) => {
  const count = await Order.countDocuments({ status: 'shipped' });
  res.send(`Shipped orders: ${count}`);
});

If there are 7 orders with status 'shipped', what will be sent in the response?

Express

app.get('/count', async (req, res) => {
  const count = await Order.countDocuments({ status: 'shipped' });
  res.send(`Shipped orders: ${count}`);
});

A Shipped orders: 7

B Shipped orders: undefined

C Shipped orders: 0

D An error because countDocuments is not a function

💡 Hint

Recall what countDocuments returns when documents match the query.

The countDocuments method returns the number of documents matching the query. Here, it returns 7, so the response is 'Shipped orders: 7'.

🧠 Conceptual

expert

Which option best explains the difference between find() and findOne() in Express MongoDB queries?

When querying MongoDB in Express, what is the key difference between find() and findOne() methods?

A <code>find()</code> requires a callback; <code>findOne()</code> only supports promises

B <code>find()</code> returns a single document; <code>findOne()</code> returns an array of documents

C <code>find()</code> modifies documents; <code>findOne()</code> only reads documents

D <code>find()</code> returns an array of all matching documents; <code>findOne()</code> returns the first matching document or null

💡 Hint

Think about the return types of these methods.

find() returns an array of all documents matching the query, while findOne() returns the first matching document or null if none found.