Encoding and decoding URLs in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of decoding a URL component?

Consider the following Node.js code snippet that decodes a URL component. What will be the output?

Node.js

const encoded = 'Hello%20World%21';
const decoded = decodeURIComponent(encoded);
console.log(decoded);

A Hello World!

B Hello%20World%21

C Hello+World!

D SyntaxError

💡 Hint

decodeURIComponent converts percent-encoded characters back to normal characters.

The string 'Hello%20World%21' is percent-encoded. decodeURIComponent converts '%20' to space and '%21' to '!'.

❓ Predict Output

intermediate

What does encodeURI do with special characters?

What is the output of this Node.js code that uses encodeURI on a URL string?

Node.js

const url = 'https://example.com/a file.html?name=John Doe&age=30';
const encoded = encodeURI(url);
console.log(encoded);

A https%3A%2F%2Fexample.com%2Fa%20file.html%3Fname%3DJohn%20Doe%26age%3D30

B https://example.com/a%20file.html%3Fname%3DJohn%20Doe%26age%3D30

C https://example.com/a%20file.html?name=John%20Doe&age=30

D https://example.com/a file.html?name=John Doe&age=30

💡 Hint

encodeURI encodes spaces but leaves URL special characters like ':' and '?' intact.

encodeURI encodes spaces as '%20' but does not encode characters like ':', '/', '?', '=', '&'.

📝 Syntax

advanced

Which option causes a SyntaxError when decoding a URL component?

Which of the following decodeURIComponent calls will cause a SyntaxError in Node.js?

A decodeURIComponent('%E0%A4%A');

B decodeURIComponent('%20');

C decodeURIComponent('%3F');

D decodeURIComponent('Hello%20World');

💡 Hint

Invalid percent-encoding sequences cause SyntaxError in decodeURIComponent.

Option A has an incomplete percent-encoded sequence '%E0%A4%A' which is invalid and causes SyntaxError.

❓ component_behavior

advanced

How does encodeURIComponent treat reserved URL characters?

Given the string 'name=John&age=30', what will encodeURIComponent produce?

Node.js

const input = 'name=John&age=30';
const output = encodeURIComponent(input);
console.log(output);

A name%3DJohn%26age=30

B name=John&age=30

C name%3DJohn&age%3D30

D name%3DJohn%26age%3D30

💡 Hint

encodeURIComponent encodes all characters except letters, digits, and a few others.

encodeURIComponent encodes '=' as '%3D' and '&' as '%26', so the entire string is encoded.

❓ state_output

expert

What is the final value of 'url' after encoding and decoding?

Consider this Node.js code. What is the final value of the variable 'url' after running all lines?

Node.js

let url = 'https://example.com/search?query=Node.js & encoding';
url = encodeURI(url);
url = decodeURIComponent(url);

A https://example.com/search?query=Node.js%20&%20encoding

B https://example.com/search?query=Node.js & encoding

C https://example.com/search?query=Node.js%20%26%20encoding

D gnidocne & sj.edoN=yreuq?hcraes/moc.elpmaxe//:sptth

💡 Hint

encodeURI encodes spaces as '%20' but leaves '&' intact; decodeURIComponent reverses encoding.

encodeURI encodes spaces as '%20' but leaves '&' as is. decodeURIComponent converts '%20' back to space. So final string matches original.

Practice

1. What does the encodeURIComponent function do in Node.js?

easy

A. It decodes an encoded URL string back to normal text.

B. It parses a URL into its components.

C. It sends an HTTP request to a URL.

D. It converts special characters in a string into a format safe for URLs.

Solution

1. Step 1: Understand the purpose of encoding URLs

   Encoding converts special characters like spaces or symbols into a safe format for URLs.

2. Step 2: Identify the role of encodeURIComponent

   This function specifically encodes parts of a URL so they can be safely transmitted.

3. Final Answer:

   It converts special characters in a string into a format safe for URLs. -> Option D

4. Quick Check:

   Encoding = safe URL string [OK]

Hint: Encoding makes URL parts safe by replacing special characters [OK]

Common Mistakes:

• Confusing encoding with decoding
• Thinking it sends HTTP requests
• Assuming it parses URLs

2. Which of the following is the correct syntax to decode a URL-encoded string in Node.js?

easy

A. decodeURIComponent('https%3A%2F%2Fexample.com')

B. decodeURI('https://example.com')

C. encodeURIComponent('https://example.com')

D. encodeURI('https%3A%2F%2Fexample.com')

Solution

1. Step 1: Identify the function to decode URL components

   decodeURIComponent is used to decode encoded parts of a URL.

2. Step 2: Check the syntax correctness

   The syntax decodeURIComponent('https%3A%2F%2Fexample.com') correctly decodes the encoded string.

3. Final Answer:

   decodeURIComponent('https%3A%2F%2Fexample.com') -> Option A

4. Quick Check:

   Decoding syntax = decodeURIComponent() [OK]

Hint: Use decodeURIComponent() to decode encoded URL parts [OK]

Common Mistakes:

• Using encode functions to decode
• Using decodeURI which decodes less strictly
• Mixing encodeURI and decodeURIComponent

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

const encoded = encodeURIComponent('Hello World!');
console.log(encoded);

medium

A. Hello%World%!

B. Hello+World!

C. Hello%20World%21

D. Hello World!

Solution

1. Step 1: Understand what encodeURIComponent does to spaces and special characters

   Spaces become %20 and exclamation marks become %21 in encoding.

2. Step 2: Apply encoding to 'Hello World!'

   The space is replaced by %20 and '!' by %21, resulting in 'Hello%20World%21'.

3. Final Answer:

   Hello%20World%21 -> Option C

4. Quick Check:

   Space = %20, ! = %21 [OK]

Hint: Spaces become %20, special chars like ! become %21 [OK]

Common Mistakes:

• Expecting spaces to become '+'
• Confusing encodeURI with encodeURIComponent
• Not encoding special characters

4. Identify the error in this Node.js code snippet:

const url = 'https://example.com/search?query=Node.js';
const encodedUrl = encodeURIComponent(url);
console.log(encodedUrl);

medium

A. No error; code works correctly.

B. Should use encodeURI instead of encodeURIComponent for full URLs.

C. Missing decodeURIComponent call after encoding.

D. encodeURIComponent cannot encode dots ('.').

Solution

1. Step 1: Understand difference between encodeURI and encodeURIComponent

   encodeURIComponent encodes all special characters, including ':' and '/', which breaks full URLs.

2. Step 2: Identify correct function for full URLs

   encodeURI is designed to encode full URLs without breaking their structure.

3. Final Answer:

   Should use encodeURI instead of encodeURIComponent for full URLs. -> Option B

4. Quick Check:

   Full URL encoding = encodeURI() [OK]

Hint: Use encodeURI for full URLs, encodeURIComponent for parts [OK]

Common Mistakes:

• Using encodeURIComponent on full URLs
• Expecting decode after encoding always
• Thinking dots cannot be encoded

5. You want to safely encode a URL query parameter that contains an email address like [e-mail]. Which code snippet correctly encodes this parameter for use in a URL?

hard

A. const param = encodeURIComponent('[e-mail]');

B. const param = encodeURI('[e-mail]').replace(/@/g, '+');

C. const param = encodeURIComponent('[e-mail]').replace(/%40/g, '@');

D. const param = encodeURI('[e-mail]');

Solution

1. Step 1: Determine correct encoding for query parameters

   Query parameters should be encoded with encodeURIComponent to encode special characters like '@'.

2. Step 2: Check each option's correctness

   const param = encodeURI('[e-mail]'); uses encodeURI which does not encode '@'. const param = encodeURI('[e-mail]').replace(/@/g, '+'); uses encodeURI and replaces '@' with '+', but '+' is interpreted as space in query strings. const param = encodeURIComponent('[e-mail]').replace(/%40/g, '@'); uses encodeURIComponent but then decodes '%40' back to '@'. const param = encodeURIComponent('[e-mail]');: const param = encodeURIComponent('[e-mail]'); correctly encodes '@' as '%40'.

3. Final Answer:

   const param = encodeURIComponent('[e-mail]'); -> Option A

4. Quick Check:

   Query param encoding = encodeURIComponent() [OK]

Hint: Use encodeURIComponent for query parameters including emails [OK]

Common Mistakes:

• Using encodeURI for query parameters
• Manually replacing encoded characters incorrectly
• Not encoding '@' in emails