Building URLs programmatically in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this URL construction code?

Consider the following Node.js code using the URL class. What will be logged to the console?

Node.js

const { URL } = require('url');
const base = 'https://example.com/path';
const url = new URL('/subpath?query=123', base);
console.log(url.href);

A https://example.com/path/subpath?query=123

B https://example.com/subpath?query=123

C https://example.com/path?query=123

D https://example.com/path/subpath

💡 Hint

Remember how the URL constructor resolves relative paths against the base URL.

When the second argument to the URL constructor starts with a slash, it replaces the path of the base URL entirely. So '/subpath' replaces '/path', resulting in 'https://example.com/subpath?query=123'.

❓ component_behavior

intermediate

How does URLSearchParams handle multiple values for the same key?

Given the following code, what will be the output of the console.log statement?

Node.js

const params = new URLSearchParams();
params.append('color', 'red');
params.append('color', 'blue');
console.log(params.toString());

A color=red,blue

B color=blue

C color=red

D color=red&color=blue

💡 Hint

URLSearchParams allows multiple values for the same key by appending them.

Appending multiple values for the same key results in repeated key-value pairs in the query string, separated by '&'.

📝 Syntax

advanced

Which option correctly creates a URL with a dynamic query parameter?

You want to create a URL 'https://api.example.com/data' with a query parameter 'id' set to a variable value. Which code snippet correctly does this?

A

const url = new URL('https://api.example.com/data');
url.searchParams.set('id', id);
console.log(url.href);

B

const url = new URL('https://api.example.com/data?id=' + id);
console.log(url.href);

C

const url = new URL('https://api.example.com/data');
url.query.id = id;
console.log(url.href);

D

const url = new URL('https://api.example.com/data');
url.search.id = id;
console.log(url.href);

💡 Hint

Use the URLSearchParams API to set query parameters dynamically.

Option A uses the URL class and its searchParams property correctly to set the query parameter. Option A concatenates strings but does not encode properly. Options A and D use invalid properties.

🔧 Debug

advanced

Why does this URL constructor code throw an error?

Examine the code below. Why does it throw a TypeError?

Node.js

const { URL } = require('url');
const base = 'https://example.com';
const url = new URL('path', base);
console.log(url.href);

A Because the base URL must be an instance of URL, not a string.

B Because the base URL is missing a trailing slash, so relative paths are invalid.

C Because 'path' is a relative path without a leading slash, it cannot be resolved against the base string.

D Because the URL constructor requires the first argument to be an absolute URL string.

💡 Hint

Check how relative paths without leading slashes are resolved against base URLs.

The URL constructor expects relative paths to start with '/' or be absolute URLs. 'path' without a leading slash is treated as a relative path to the current directory, which is invalid here and causes a TypeError.

🧠 Conceptual

expert

What is the effect of setting url.hash in a URL object?

Given the code below, what will be the value of url.href after setting url.hash = '#section2'?

Node.js

const { URL } = require('url');
const url = new URL('https://example.com/page?query=1');
url.hash = '#section2';
console.log(url.href);

A https://example.com/page?query=1#section2

B https://example.com/page#section2?query=1

C https://example.com/page?query=1

D https://example.com/page#section2

💡 Hint

The hash part of a URL comes after the query string and starts with '#'.

Setting url.hash adds or replaces the fragment identifier at the end of the URL, after the query string.

Practice

1. What is the main benefit of using the URL class in Node.js when building URLs programmatically?

easy

A. It automatically encodes special characters and manages URL parts safely.

B. It allows direct string concatenation without any checks.

C. It disables query parameters to simplify URLs.

D. It only works with HTTP URLs, ignoring others.

Solution

1. Step 1: Understand the purpose of the URL class

   The URL class helps build URLs by handling encoding and structure automatically.

2. Step 2: Compare options with URL class features

   Only It automatically encodes special characters and manages URL parts safely. correctly describes automatic encoding and safe URL part management.

3. Final Answer:

   It automatically encodes special characters and manages URL parts safely. -> Option A

4. Quick Check:

   URL class = safe URL building [OK]

Hint: Remember: URL class handles encoding and structure safely [OK]

Common Mistakes:

• Thinking URL class just concatenates strings
• Assuming URL class disables query parameters
• Believing URL class only supports HTTP URLs

2. Which of the following is the correct way to create a new URL object for 'https://example.com' in Node.js?

easy

A. const url = new URL('https://example.com');

B. const url = new URL('example.com');

C. const url = URL('https://example.com');

D. const url = new URL(); url.href = 'https://example.com';

Solution

1. Step 1: Check URL constructor usage

   The URL constructor requires a full URL string including protocol, e.g., 'https://example.com'.

2. Step 2: Validate each option

   const url = new URL('https://example.com'); correctly uses new URL with full URL string. const url = new URL('example.com'); misses protocol, C misses 'new', D uses empty constructor which is invalid.

3. Final Answer:

   const url = new URL('https://example.com'); -> Option A

4. Quick Check:

   Use new URL('full-url') syntax [OK]

Hint: Always include protocol and use 'new' with URL [OK]

Common Mistakes:

• Omitting 'new' keyword
• Passing URL without protocol
• Trying to create URL without constructor

3. What will be the output of this Node.js code?

const url = new URL('https://example.com/path');
url.searchParams.append('q', 'nodejs');
url.searchParams.append('page', '2');
console.log(url.toString());

medium

A. https://example.com/path?page=2&q=nodejs

B. https://example.com/path?q=nodejs&page=2

C. https://example.com/path?q=nodejs&page=2&

D. https://example.com/path

Solution

1. Step 1: Understand searchParams.append behavior

   Appending 'q=nodejs' then 'page=2' adds these query parameters in order.

2. Step 2: Check URL string output

   Calling toString() returns full URL with query string '?q=nodejs&page=2' appended to path.

3. Final Answer:

   https://example.com/path?q=nodejs&page=2 -> Option B

4. Quick Check:

   Appended params appear in order in URL string [OK]

Hint: searchParams.append adds parameters in order [OK]

Common Mistakes:

• Assuming parameters order is reversed
• Expecting trailing '&' at end
• Ignoring appended query parameters

4. Identify the error in this code snippet that tries to add a query parameter:

const url = new URL('https://example.com');
url.searchParams.add('key', 'value');
console.log(url.href);

medium

A. The 'href' property is read-only and cannot be logged.

B. URL constructor is missing the protocol.

C. You cannot modify searchParams after URL creation.

D. The method 'add' does not exist on searchParams; should use 'append'.

Solution

1. Step 1: Check searchParams methods

   The correct method to add a query parameter is 'append', not 'add'.

2. Step 2: Validate other code parts

   URL has protocol, searchParams can be modified, and href can be logged. Only method name is wrong.

3. Final Answer:

   The method 'add' does not exist on searchParams; should use 'append'. -> Option D

4. Quick Check:

   Use searchParams.append, not add [OK]

Hint: Use 'append' to add query parameters, not 'add' [OK]

Common Mistakes:

• Using 'add' instead of 'append'
• Thinking href is not accessible
• Believing searchParams is immutable

5. You want to build a URL with base 'https://api.example.com/search' and add parameters 'query' with value 'node.js tips' and 'sort' with value 'recent'. Which code correctly builds this URL with proper encoding?

hard

A. const url = new URL('https://api.example.com/search');
url.searchParams.set('query', 'node.js tips');
url.searchParams.set('sort', 'recent');
console.log(url.toString());

B. const url = 'https://api.example.com/search?query=node.js tips&sort=recent';
console.log(url);

C. const url = new URL('https://api.example.com/search');
url.searchParams.append('query', 'node.js tips');
url.searchParams.append('sort', 'recent');
console.log(url.href);

D. const url = new URL('https://api.example.com/search');
url.searchParams.add('query', 'node.js tips');
url.searchParams.add('sort', 'recent');
console.log(url.href);

Solution

1. Step 1: Understand URL and searchParams usage

   Use new URL with base URL, then append parameters with searchParams.append to add multiple values safely.

2. Step 2: Check encoding and method correctness

   const url = new URL('https://api.example.com/search'); url.searchParams.append('query', 'node.js tips'); url.searchParams.append('sort', 'recent'); console.log(url.href); uses append correctly and prints href, which includes proper encoding of spaces as '%20'. const url = new URL('https://api.example.com/search'); url.searchParams.set('query', 'node.js tips'); url.searchParams.set('sort', 'recent'); console.log(url.toString()); uses set which replaces values but also works; however, question asks for correct code with proper encoding and append is more common for adding multiple params. const url = 'https://api.example.com/search?query=node.js tips&sort=recent'; console.log(url); is manual string and misses encoding. const url = new URL('https://api.example.com/search'); url.searchParams.add('query', 'node.js tips'); url.searchParams.add('sort', 'recent'); console.log(url.href); uses non-existent 'add' method.

3. Final Answer:

   Option C code correctly builds URL with proper encoding and appends parameters. -> Option C

4. Quick Check:

   Use URL + searchParams.append for safe, encoded URLs [OK]

Hint: Use URL and searchParams.append for safe, encoded query parameters [OK]

Common Mistakes:

• Manually concatenating query strings without encoding
• Using non-existent 'add' method
• Ignoring encoding of spaces and special characters