Query string parsing in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does Express parse query strings by default?

Consider an Express route handler receiving a URL with query parameters like ?name=John&age=30. What will req.query contain?

Express

app.get('/user', (req, res) => {
  res.json(req.query);
});

A {"name": "John", "age": "30"}

B {"name": "John", "age": 30}

C ["name", "John", "age", "30"]

D An empty object {}

💡 Hint

Express parses query strings into an object with string values by default.

Express uses the querystring module by default, which parses query parameters into an object with all values as strings.

📝 Syntax

intermediate

Which code correctly parses nested query strings in Express?

You want to parse a query string like ?user[name]=Alice&user[age]=25 into a nested object. Which Express setup achieves this?

A app.use(express.urlencoded({ extended: false }));

B app.use(express.urlencoded({ extended: true }));

C app.use(express.json());

D app.use(express.query({ extended: true }));

💡 Hint

Look for middleware that supports nested objects in URL-encoded data.

The express.urlencoded({ extended: true }) middleware uses the qs library which supports nested objects in query strings.

🔧 Debug

advanced

Why does req.query not parse arrays as expected?

Given the URL ?colors=red&colors=blue, req.query.colors is a string instead of an array. What causes this behavior?

Express

app.get('/colors', (req, res) => {
  res.json(req.query.colors);
});

A The server needs to use express.json() middleware to parse arrays.

B The URL is malformed; arrays require brackets like colors[]=red&colors[]=blue.

C Express default query parser returns the last value as a string, not an array.

D req.query is always a string regardless of query parameters.

💡 Hint

Express default parser does not convert repeated keys into arrays.

Express's default query parser returns the last value for repeated keys as a string. To get arrays, the query string must use bracket notation or a custom parser.

🧠 Conceptual

advanced

What is the effect of using a custom query parser in Express?

If you configure Express with app.set('query parser', 'simple') or app.set('query parser', 'extended'), how does it affect req.query?

A Both options produce identical parsing results.

B 'simple' parses JSON strings; 'extended' parses XML strings.

C 'simple' disables query parsing; 'extended' enables it.

D 'simple' uses the built-in querystring module; 'extended' uses qs for nested parsing.

💡 Hint

Express supports two query parsers with different capabilities.

The 'simple' parser uses Node's querystring module which does not support nested objects. The 'extended' parser uses the qs library which supports nested objects and arrays.

❓ state_output

expert

What is the output of this Express route with complex query string?

Given the route below and the URL /search?filters[price][min]=10&filters[price][max]=100&tags=red&tags=blue, what is the JSON response?

Express

app.use(express.urlencoded({ extended: true }));
app.get('/search', (req, res) => {
  res.json(req.query);
});

A {"filters": {"price": {"min": "10", "max": "100"}}, "tags": ["red", "blue"]}

B {"filters": {"price": {"min": "10", "max": "100"}}, "tags": "blue"}

C {"filters": {"price": {"min": 10, "max": 100}}, "tags": "blue"}

D {"filters": {}, "tags": ["red", "blue"]}

💡 Hint

express.urlencoded({ extended: true }) parses nested objects and repeated keys as arrays.

The extended: true option uses the qs library which parses nested objects and repeated keys into arrays. All values are strings.

Practice

1. In Express, which object lets you access the query parameters from a URL like /search?term=books?

easy

A. req.query

B. req.params

C. req.body

D. req.url

Solution

1. Step 1: Understand Express request objects

   Express provides different objects to access parts of the request: req.params for route params, req.body for POST data, and req.query for query strings.

2. Step 2: Identify query string access

   The query string is the part after ? in a URL. Express parses it into req.query as an object.

3. Final Answer:

   req.query -> Option A

4. Quick Check:

   Query string = req.query [OK]

Hint: Query strings are in req.query object in Express [OK]

Common Mistakes:

• Confusing req.params with req.query
• Trying to read query from req.body
• Using req.url directly without parsing

2. Which of the following is the correct way to read a query parameter named page in an Express route handler?

easy

A. const page = req.query.page;

B. const page = req.body.page;

C. const page = req.params.page;

D. const page = req.url.page;

Solution

1. Step 1: Recall query parameter access syntax

   Query parameters are accessed via req.query as key-value pairs.

2. Step 2: Match correct syntax

   To get the page parameter, use req.query.page. Other options access wrong objects.

3. Final Answer:

   const page = req.query.page; -> Option A

4. Quick Check:

   Query param 'page' = req.query.page [OK]

Hint: Use req.query.paramName to get query parameters [OK]

Common Mistakes:

• Using req.params for query parameters
• Trying to read query from req.body
• Accessing req.url.page which is undefined

3. Given this Express route handler:

app.get('/items', (req, res) => {
  const limit = Number(req.query.limit) || 10;
  res.send(`Limit is ${limit}`);
});

What will be the response if the URL is /items?limit=5?

medium

A. "Limit is 10"

B. "Limit is undefined"

C. "Limit is 5"

D. "Limit is NaN"

Solution

1. Step 1: Convert query string value to number

   The code uses Number(req.query.limit) to convert the string '5' to number 5.

2. Step 2: Use default if conversion fails

   If conversion results in 0 or NaN, it defaults to 10. Here, 5 is valid, so limit = 5.

3. Final Answer:

   "Limit is 5" -> Option C

4. Quick Check:

   Number('5') = 5, so output = "Limit is 5" [OK]

Hint: Convert query strings to number before use [OK]

Common Mistakes:

• Not converting string to number
• Assuming req.query.limit is a number
• Ignoring default fallback logic

4. Consider this Express route:

app.get('/search', (req, res) => {
  const term = req.query.term.toLowerCase();
  res.send(term);
});

What error will occur if the URL is /search (no query parameter)?

medium

A. SyntaxError: Unexpected token

B. TypeError: Cannot read property 'toLowerCase' of undefined

C. ReferenceError: term is not defined

D. No error, response is empty string

Solution

1. Step 1: Check for missing query parameter

   If term is missing, req.query.term is undefined.

2. Step 2: Calling method on undefined causes error

   Calling toLowerCase() on undefined throws a TypeError.

3. Final Answer:

   TypeError: Cannot read property 'toLowerCase' of undefined -> Option B

4. Quick Check:

   Undefined.toLowerCase() throws TypeError [OK]

Hint: Check if query param exists before using methods [OK]

Common Mistakes:

• Assuming query param always exists
• Not handling undefined values
• Expecting empty string instead of error

5. You want to handle a query parameter sort that can be either asc or desc. If missing or invalid, default to asc. Which code snippet correctly implements this in Express?

hard

A. const sort = req.query.sort === 'asc' ? 'asc' : req.query.sort === 'desc' ? 'desc' : 'desc';

B. const sort = req.query.sort || 'asc';

C. const sort = req.query.sort === 'asc' ? 'asc' : 'desc';

D. const sort = ['asc', 'desc'].includes(req.query.sort) ? req.query.sort : 'asc';

Solution

1. Step 1: Validate query parameter value

   We must check if req.query.sort is either 'asc' or 'desc'. Using includes() on an array is a clean way.

2. Step 2: Provide default if invalid or missing

   If the value is not in the allowed list, default to 'asc'. This avoids invalid values.

3. Final Answer:

   const sort = ['asc', 'desc'].includes(req.query.sort) ? req.query.sort : 'asc'; -> Option D

4. Quick Check:

   Check allowed values with includes() and default [OK]

Hint: Use includes() to validate query param values before use [OK]

Common Mistakes:

• Not validating allowed values
• Using simple || which accepts invalid strings
• Overcomplicating with nested ternaries