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.