Reading and writing buffer data in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Buffer write and read code?

Consider the following Node.js code that writes a string to a buffer and then reads it back. What will be logged to the console?

Node.js

const buf = Buffer.alloc(10);
buf.write('hello');
console.log(buf.toString('utf8', 0, 5));

A "\u0000\u0000\u0000\u0000\u0000"

B "hello\u0000"

C "hellohello"

D "hello"

💡 Hint

Remember that Buffer.alloc creates a buffer filled with zeros and write returns the number of bytes written.

The code writes 'hello' into the buffer starting at index 0. Then it reads the first 5 bytes as a UTF-8 string, which matches 'hello'. The rest of the buffer remains zeros but is not included in the output.

❓ component_behavior

intermediate

What happens when you slice a Buffer and modify the slice?

Given this code, what will be the output after modifying the slice?

Node.js

const buf = Buffer.from('abcdef');
const slice = buf.slice(1, 4);
slice[0] = 120;
console.log(buf.toString());

A "abxdef"

B "abcdef"

C "xbcdef"

D "axcdef"

💡 Hint

Buffer slices share the same memory as the original buffer.

The slice covers indices 1 to 3 (characters 'b', 'c', 'd'). Changing slice[0] changes buf[1] to 120 (which is 'x'). So the original buffer becomes 'a', 'x', 'c', 'd', 'e', 'f'.

📝 Syntax

advanced

Which option correctly creates a Buffer from a hex string?

You want to create a Buffer from the hex string '48656c6c6f'. Which code snippet does this correctly?

A Buffer.from('48656c6c6f', 'utf8')

B Buffer.from('48656c6c6f', 'hex')

C Buffer.alloc('48656c6c6f', 'hex')

D Buffer.allocUnsafe('48656c6c6f')

💡 Hint

The encoding parameter tells Buffer how to interpret the string.

Buffer.from with 'hex' encoding interprets the string as hexadecimal bytes. Using 'utf8' treats it as normal text, which is incorrect here. Buffer.alloc and allocUnsafe do not accept string data as input.

🔧 Debug

advanced

Why does this Buffer write return 0 bytes written?

Look at this code snippet. Why does buf.write return 0?

Node.js

const buf = Buffer.alloc(5);
const bytesWritten = buf.write('hello world', 0, 5, 'utf8');
console.log(bytesWritten);

A Because the string is longer than the buffer size, so nothing is written.

B Because the length parameter is the max bytes to write, but the string is truncated and no bytes fit.

C Because the length parameter is the max bytes to write, but the string is longer and write returns the number of bytes actually written.

D Because the length parameter is less than the string length, so it writes only 5 bytes.

💡 Hint

Check the meaning of the length parameter in buf.write.

The length parameter limits how many bytes to write. The string 'hello world' is longer than 5 bytes, so only the first 5 bytes are written. The function returns the number of bytes written, which is 5, not 0. So the premise is wrong; the code returns 5, not 0.

❓ state_output

expert

What is the final content of the buffer after these operations?

Analyze the code and determine the final string content of the buffer.

Node.js

const buf = Buffer.alloc(6);
buf.write('abc');
buf.write('def', 3);
buf.fill('x', 1, 4);
console.log(buf.toString());

A "axxxef"

B "axxxxf"

C "abcxxx"

D "abcdef"

💡 Hint

Remember that fill replaces bytes in the specified range, and write overwrites bytes starting at the given offset.

First, 'abc' is written at index 0: [a,b,c,?, ?, ?]. Then 'def' is written at index 3: [a,b,c,d,e,f]. Then fill replaces bytes from index 1 to 4 (excluding 4) with 'x': indices 1,2,3 become 'x'. Final buffer: [a,x,x,x,e,f].