Creating buffers in Node.js - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Node.js buffer code?

Consider the following code snippet that creates a buffer and writes a string into it. What will be logged to the console?

Node.js

const buf = Buffer.alloc(5);
buf.write('HelloWorld');
console.log(buf.toString());

A Hello

B Error: Buffer overflow

C Hello\u0000

D HelloWorld

💡 Hint

Buffer.alloc creates a fixed size buffer. Writing beyond its size truncates the string.

Buffer.alloc(5) creates a buffer of length 5. Writing 'HelloWorld' writes only the first 5 characters 'Hello'. The rest is ignored. So buf.toString() outputs 'Hello'.

❓ Predict Output

intermediate

What does this Buffer.from() call produce?

What will be the output of this code snippet?

Node.js

const buf = Buffer.from([65, 66, 67, 68]);
console.log(buf.toString());

A ABCD

B [65,66,67,68]

C 65666768

D Error: Invalid argument

💡 Hint

Buffer.from with an array creates a buffer with those byte values.

The array [65,66,67,68] corresponds to ASCII codes for 'A', 'B', 'C', 'D'. Buffer.from creates a buffer with those bytes. toString() converts bytes to string 'ABCD'.

❓ Predict Output

advanced

What error does this buffer code raise?

What error will this code produce when run?

Node.js

const buf = Buffer.allocUnsafe(3);
buf.write('NodeJS', 0, 10);
console.log(buf.toString());

A SyntaxError: Unexpected token

B TypeError: Invalid argument

C No error, outputs 'NodeJS'

D RangeError: Attempt to write outside buffer bounds

💡 Hint

Writing more bytes than buffer size causes a range error.

Buffer.allocUnsafe(3) creates a buffer of length 3. Trying to write 10 bytes starting at 0 exceeds buffer size, causing a RangeError.

❓ component_behavior

advanced

How does Buffer.concat behave with empty buffers?

Given the following code, what will be the output?

Node.js

const buf1 = Buffer.from('Hello');
const buf2 = Buffer.alloc(0);
const buf3 = Buffer.from('World');
const result = Buffer.concat([buf1, buf2, buf3]);
console.log(result.toString());

A Hello\u0000World

B HelloWorld

C Hello

D World

💡 Hint

Buffer.concat ignores empty buffers and joins the rest.

Buffer.concat joins all buffers in order. Empty buffers add no content. So the result is 'Hello' + '' + 'World' = 'HelloWorld'.

📝 Syntax

expert

Which option correctly creates a zero-filled buffer of length 4?

Select the code snippet that creates a buffer of length 4 filled with zeros without any warnings or errors.

A const buf = new Buffer(4);

B const buf = Buffer.from(4);

C const buf = Buffer.alloc(4);

D const buf = Buffer.allocUnsafe(4).fill(0);

💡 Hint

Buffer.alloc creates zero-filled buffers safely.

Buffer.alloc(4) creates a zero-filled buffer of length 4 safely. Buffer.from(4) is invalid. Buffer.allocUnsafe(4) creates uninitialized buffer, but fill(0) fills it with zeros (valid but less preferred). new Buffer(4) is deprecated and unsafe.

Practice

1. What does Buffer.alloc(10) do in Node.js?

easy

A. Creates a buffer of 10 bytes filled with zeros

B. Creates a buffer of 10 bytes filled with random data

C. Creates a buffer from a string of length 10

D. Creates an empty array of length 10

Solution

1. Step 1: Understand Buffer.alloc usage

   Buffer.alloc(size) creates a buffer of the given size filled with zeros.

2. Step 2: Analyze the argument 10

   The argument 10 means the buffer will have 10 bytes, all initialized to zero.

3. Final Answer:

   Creates a buffer of 10 bytes filled with zeros -> Option A

4. Quick Check:

   Buffer.alloc(10) = zero-filled buffer [OK]

Hint: Buffer.alloc creates zero-filled buffer of given size [OK]

Common Mistakes:

• Thinking Buffer.alloc fills with random data
• Confusing Buffer.alloc with Buffer.from
• Assuming it creates an empty array

2. Which of the following is the correct syntax to create a buffer from the string 'hello'?

easy

A. Buffer.alloc('hello')

B. Buffer.from('hello')

C. new Buffer('hello')

D. Buffer.create('hello')

Solution

1. Step 1: Recall correct Buffer creation methods

   In modern Node.js, Buffer.from(string) creates a buffer from a string.

2. Step 2: Check options

   Buffer.alloc expects a size number, new Buffer is deprecated, and Buffer.create does not exist.

3. Final Answer:

   Buffer.from('hello') -> Option B

4. Quick Check:

   Buffer.from(string) creates buffer from string [OK]

Hint: Use Buffer.from(string) to create buffer from text [OK]

Common Mistakes:

• Using Buffer.alloc with string argument
• Using deprecated new Buffer() constructor
• Assuming Buffer.create exists

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

const buf = Buffer.from('abc');
console.log(buf.length);

medium

A. Error

B. 6

C. undefined

D. 3

Solution

1. Step 1: Create buffer from string 'abc'

   Buffer.from('abc') creates a buffer with bytes representing 'a', 'b', 'c'.

2. Step 2: Check buffer length

   The length property returns the number of bytes, which is 3 for 'abc'.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Buffer length of 'abc' = 3 [OK]

Hint: Buffer length equals number of bytes in string [OK]

Common Mistakes:

• Assuming length is number of characters times 2
• Expecting undefined or error
• Confusing length with string length property

4. Identify the error in this code snippet:

const buf = Buffer.alloc('5');
console.log(buf.length);

medium

A. Buffer.alloc does not have length property

B. Buffer.alloc cannot be used without 'new'

C. Buffer.alloc expects a number, not a string

D. No error, code runs fine

Solution

1. Step 1: Check Buffer.alloc argument type

   Buffer.alloc expects a number for size, but '5' is a string.

2. Step 2: Understand type coercion in Buffer.alloc

   Passing a string causes a TypeError because size must be a number.

3. Final Answer:

   Buffer.alloc expects a number, not a string -> Option C

4. Quick Check:

   Buffer.alloc('5') causes type error [OK]

Hint: Pass number, not string, to Buffer.alloc size [OK]

Common Mistakes:

• Using string instead of number for size
• Thinking Buffer.alloc needs 'new' keyword
• Assuming length property is missing

5. You want to create a buffer from an array of bytes [72, 101, 108, 108, 111] representing 'Hello'. Which code correctly creates this buffer and converts it back to a string?

hard

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

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

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

D. const buf = Buffer.alloc(5); buf.write([72,101,108,108,111]); console.log(buf.toString());

Solution

1. Step 1: Create buffer from array of bytes

   Buffer.from(array) creates a buffer from an array of byte values correctly.

2. Step 2: Convert buffer back to string

   buf.toString() converts the buffer bytes to the string 'Hello'.

3. Step 3: Analyze other options

   Using Buffer.alloc([72,101,108,108,111]) is wrong because alloc expects a number. Using Buffer.from('72,101,108,108,111') creates a buffer from the string of numbers, not bytes. Using buf.write([72,101,108,108,111]) is invalid as write doesn't accept arrays directly.

4. Final Answer:

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

5. Quick Check:

   Buffer.from(array) + toString() = 'Hello' [OK]

Hint: Use Buffer.from(array) to create buffer from bytes [OK]

Common Mistakes:

• Using Buffer.alloc with array argument
• Passing string of numbers instead of array
• Trying to write array directly into buffer