Parsing query strings in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of parsing a simple query string?

Given the following Node.js code using the built-in URLSearchParams, what will be logged to the console?

Node.js

const queryString = 'name=alice&age=30';
const params = new URLSearchParams(queryString);
console.log(params.get('name'), params.get('age'));

A "alice" "30"

B "name" "age"

C undefined undefined

D SyntaxError

💡 Hint

Think about how URLSearchParams extracts values by key.

URLSearchParams parses the query string and allows access to values by their keys using get(). Here, 'name' returns 'alice' and 'age' returns '30'.

❓ Predict Output

intermediate

How does URLSearchParams handle repeated keys?

What will the following code output when parsing a query string with repeated keys?

Node.js

const queryString = 'color=red&color=blue&color=green';
const params = new URLSearchParams(queryString);
console.log(params.get('color'));

A "green"

B "red,blue,green"

C "red"

D TypeError

💡 Hint

URLSearchParams.get returns the first value for the key.

When keys repeat, get() returns the first value only. To get all values, use getAll().

🔧 Debug

advanced

Why does this code throw an error when parsing a query string?

Consider this code snippet. Why does it throw a TypeError?

Node.js

const queryString = 'foo=bar&baz';
const params = new URLSearchParams(queryString);
console.log(params.get('baz'));

A Because URLSearchParams requires all keys to have values, missing '=' causes TypeError.

B Because 'baz' is an invalid key and causes SyntaxError.

C Because get() method is not defined on URLSearchParams.

D Because 'baz' has no '=' sign and no value, get('baz') returns an empty string, not a TypeError.

💡 Hint

Check how URLSearchParams treats keys without values.

URLSearchParams treats keys without '=' as keys with empty string values. get('baz') returns an empty string, not an error.

❓ component_behavior

advanced

What is the behavior of URLSearchParams.append() with existing keys?

Given this code, what will be the output of params.toString()?

Node.js

const params = new URLSearchParams('key=value1');
params.append('key', 'value2');
console.log(params.toString());

A "key=value2"

B "key=value1&key=value2"

C "key=value1,value2"

D "key=value1"

💡 Hint

append() adds a new value for the same key without replacing existing ones.

append() adds another value for the same key, so the query string has two key=value pairs.

📝 Syntax

expert

Which option correctly parses and modifies a query string to add a new parameter?

Select the code snippet that correctly parses 'a=1&b=2', adds 'c=3', and outputs the updated query string.

A const params = new URLSearchParams('a=1&b=2'); params.append('c', 3); console.log(params.toString());

B const params = new URLSearchParams('a=1&b=2'); params.add('c', 3); console.log(params.toString());

C const params = new URLSearchParams('a=1&b=2'); params.push('c', 3); console.log(params.toString());

D const params = new URLSearchParams('a=1&b=2'); params.set('c', 3); console.log(params.toString());

💡 Hint

Check the correct method name to add a new key-value pair without replacing existing keys.

append() adds a new key-value pair. set() replaces existing key's value. add() and push() are not valid methods.