Query string parsing in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the query parameter named 'name' from the request.

Express

app.get('/hello', (req, res) => {
  const name = req.query.[1];
  res.send(`Hello, ${name}!`);
});

Drag options to blanks, or click blank then click option'

Aquery

Bparams

Cbody

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using req.params instead of req.query

Trying to access req.body for query parameters

The query parameters are accessed via req.query. To get the value of the 'name' parameter, use req.query.name.

2fill in blank

medium

Complete the code to send back the value of the 'age' query parameter as a number.

Express

app.get('/age', (req, res) => {
  const age = Number(req.query.[1]);
  res.send(`Your age is ${age}`);
});

Drag options to blanks, or click blank then click option'

Ayears

Bage

CuserAge

DageParam

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong parameter name

Not converting the string to a number

The query parameter is named 'age', so use req.query.age. Wrapping it with Number() converts the string to a number.

3fill in blank

hard

Fix the error in accessing the 'search' query parameter correctly.

Express

app.get('/search', (req, res) => {
  const term = req.[1].search;
  res.send(`Searching for: ${term}`);
});

Drag options to blanks, or click blank then click option'

Aquery

Bbody

Cparams

Dheaders

Attempts:

3 left

💡 Hint

Common Mistakes

Using req.params instead of req.query

Trying to get query parameters from req.body

Query parameters are accessed via req.query. Using req.params or others will not get query string values.

4fill in blank

hard

Fill both blanks to parse 'page' and 'limit' query parameters as numbers.

Express

app.get('/items', (req, res) => {
  const page = Number(req.query.[1]);
  const limit = Number(req.query.[2]);
  res.send(`Page: ${page}, Limit: ${limit}`);
});

Drag options to blanks, or click blank then click option'

Apage

Blimit

Ccount

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter names

Not converting to numbers

The query parameters are named 'page' and 'limit'. Use req.query.page and req.query.limit and convert them to numbers.

5fill in blank

hard

Fill all three blanks to parse 'sort', 'order', and 'filter' query parameters and send a summary.

Express

app.get('/data', (req, res) => {
  const sort = req.query.[1];
  const order = req.query.[2];
  const filter = req.query.[3];
  res.send(`Sort by: ${sort}, Order: ${order}, Filter: ${filter}`);
});

Drag options to blanks, or click blank then click option'

Asort

Border

Cfilter

Dtype

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names

Mixing parameter names with other properties

Use the exact query parameter names: 'sort', 'order', and 'filter' to access their values from req.query.

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