Buffer to string conversion in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Node.js Buffer to string conversion?

Consider the following code snippet that converts a buffer to a string. What will be printed to the console?

Node.js

const buf = Buffer.from([72, 101, 108, 108, 111]);
console.log(buf.toString());

A "undefined"

B "[72,101,108,108,111]"

C "Buffer(5)"

D "Hello"

💡 Hint

Think about what the buffer contains and what toString() does by default.

The buffer contains ASCII codes for the letters H, e, l, l, o. Calling toString() converts these codes to their character equivalents, resulting in the string "Hello".

❓ Predict Output

intermediate

What does this code output when specifying encoding?

What will this code print to the console?

Node.js

const buf = Buffer.from('68656c6c6f', 'hex');
console.log(buf.toString('utf8'));

A "hello"

B "68656c6c6f"

C "undefined"

D "\u6865\u6c6c\u6f"

💡 Hint

The buffer is created from hex values representing ASCII characters.

The buffer is created from the hex string representing the ASCII codes for "hello". Converting it to a UTF-8 string prints "hello".

❓ component_behavior

advanced

What happens if you convert a buffer with invalid UTF-8 bytes to string?

Given this buffer with invalid UTF-8 bytes, what will be the output of toString()?

Node.js

const buf = Buffer.from([0xff, 0xfe, 0xfd]);
console.log(buf.toString('utf8'));

A "\xff\xfe\xfd"

B Throws a SyntaxError

C "���"

D "" (empty string)

💡 Hint

Invalid UTF-8 bytes are replaced with a special character when converted to string.

Invalid UTF-8 bytes are replaced by the Unicode replacement character � (U+FFFD) when converted to string, so the output is three replacement characters.

📝 Syntax

advanced

Which option correctly converts a buffer to a base64 string?

Select the code snippet that correctly converts a buffer to a base64 encoded string.

A const b64 = buf.toString('base64');

B const b64 = buf.encode('base64');

C const b64 = Buffer.toString(buf, 'base64');

D const b64 = buf.toBase64();

💡 Hint

Check the Buffer API for the correct method and parameter to convert to base64.

The toString method accepts an encoding parameter like 'base64' to convert the buffer to a base64 string. Other methods do not exist or are incorrect.

🔧 Debug

expert

Why does this buffer to string conversion produce unexpected output?

This code is intended to convert a buffer to a UTF-8 string, but the output is garbled. What is the most likely cause?

Node.js

const buf = Buffer.from('c3a9', 'utf8');
console.log(buf.toString());

A Buffer.from() cannot accept strings, only arrays

B The input string is hex but encoded as UTF-8, causing wrong bytes in buffer

C toString() requires an encoding parameter to decode properly

D The buffer is empty, so output is empty string

💡 Hint

Consider what the input string represents and how Buffer.from interprets it with the given encoding.

The string 'c3a9' is a hex representation but is passed as a UTF-8 string, so the buffer contains bytes for characters 'c', '3', 'a', '9' instead of the intended hex bytes. This causes garbled output.

Practice

1. What does the toString() method do when called on a Node.js Buffer?

easy

A. Changes the buffer data to uppercase letters

B. Deletes the buffer data permanently

C. Creates a new buffer with double the size

D. Converts the raw buffer data into a readable string using an encoding

Solution

1. Step 1: Understand Buffer data

   A Buffer holds raw binary data that is not human-readable.

2. Step 2: Role of toString()

   The toString() method converts this raw data into a readable string using a specified encoding, defaulting to UTF-8.

3. Final Answer:

   Converts the raw buffer data into a readable string using an encoding -> Option D

4. Quick Check:

   Buffer.toString() = readable string [OK]

Hint: Remember: toString() makes buffer data human-readable [OK]

Common Mistakes:

• Thinking toString() deletes or modifies buffer data
• Confusing buffer size changes with toString()
• Assuming toString() changes letter case

2. Which of the following is the correct syntax to convert a Buffer named buf to a string using ASCII encoding?

easy

A. buf.toString('ascii')

B. buf.toString(ascii)

C. buf.toString[ascii]

D. buf.toString{ascii}

Solution

1. Step 1: Check method syntax

   The toString() method takes an optional encoding as a string argument inside parentheses.

2. Step 2: Validate correct usage

   Passing the encoding as a string literal like 'ascii' inside parentheses is correct syntax.

3. Final Answer:

   buf.toString('ascii') -> Option A

4. Quick Check:

   toString('encoding') uses quotes and parentheses [OK]

Hint: Encoding must be a string inside parentheses [OK]

Common Mistakes:

• Omitting quotes around encoding
• Using square or curly brackets instead of parentheses
• Passing encoding as a variable without quotes

3. What will be the output of this code?

const buf = Buffer.from('48656c6c6f', 'hex');
console.log(buf.toString());

medium

A. 48656c6c6f

B. Hello

C. Error: Invalid buffer

D. undefined

Solution

1. Step 1: Create buffer from hex string

   The buffer contains bytes representing the hex values for characters: 48='H', 65='e', 6c='l', 6c='l', 6f='o'.

2. Step 2: Convert buffer to string

   Calling toString() without encoding defaults to UTF-8, decoding bytes to 'Hello'.

3. Final Answer:

   Hello -> Option B

4. Quick Check:

   Buffer.from(hex).toString() = decoded text [OK]

Hint: Hex buffer toString() shows decoded text, not hex [OK]

Common Mistakes:

• Expecting output to be the hex string itself
• Assuming toString() throws error on hex buffers
• Confusing buffer content with string representation

4. Identify the error in this code snippet:

const buf = Buffer.from('hello');
const str = buf.toString(utf8);
console.log(str);

medium

A. toString() cannot convert buffers

B. Buffer.from() requires encoding argument

C. utf8 should be a string: 'utf8'

D. console.log() is missing parentheses

Solution

1. Step 1: Check toString() argument

   The encoding argument must be a string literal, so it should be 'utf8' with quotes.

2. Step 2: Identify error cause

   Passing utf8 without quotes causes a ReferenceError because utf8 is undefined as a variable.

3. Final Answer:

   utf8 should be a string: 'utf8' -> Option C

4. Quick Check:

   Encoding must be quoted string [OK]

Hint: Always quote encoding names in toString() [OK]

Common Mistakes:

• Forgetting quotes around encoding
• Thinking Buffer.from() always needs encoding
• Misreading console.log syntax

5. You have a Buffer buf containing UTF-8 encoded text. How do you convert only the first 5 bytes to a string?

hard

A. buf.toString('utf8', 0, 5)

B. buf.toString(0, 5)

C. buf.toString('utf8').slice(0, 5)

D. buf.toString(5)

Solution

1. Step 1: Understand toString() parameters

   The toString() method can take encoding, start, and end byte positions.

2. Step 2: Use correct parameter order

   To convert first 5 bytes, call toString('utf8', 0, 5) specifying encoding and byte range.

3. Final Answer:

   buf.toString('utf8', 0, 5) -> Option A

4. Quick Check:

   toString(encoding, start, end) slices buffer [OK]

Hint: Use toString with encoding and byte range [OK]

Common Mistakes:

• Omitting encoding argument
• Using slice on string instead of buffer
• Passing wrong parameter order