Buffers & Binary Data

A Buffer is Node's fixed-length container for raw bytes, the building block it uses to handle files, network data, and any binary content that doesn't fit neatly into a text string.

Learn Buffers & Binary Data in our free Node.js course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Node.js course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end of this lesson you'll be able to create buffers with from and alloc, convert between bytes and text in utf8, hex, and base64, read individual bytes, join buffers with concat, and understand why stream chunks arrive as Buffers.

What You'll Learn in This Lesson

1️⃣ Creating Buffers

There are two main ways to make a buffer. Buffer.from(string) takes text and encodes it as bytes (utf8 by default). Buffer.alloc(n) makes an empty, zero-filled buffer of n bytes that you can fill in yourself.

When you log a buffer, Node shows its bytes as hex (like ). You can index it like an array to read one byte, ask for its .length in bytes, and call .toString('utf8') to read it back as text.

2️⃣ Encodings: utf8, hex & base64

The same bytes can be displayed in different encodings. .toString('hex') shows each byte as two hex digits, handy for debugging. .toString('base64') packs the bytes into a compact, text-safe form often used for tokens, data URLs, and email attachments.

Going the other way, Buffer.from(str, 'base64') (or 'hex' ) decodes an encoded string back into the original bytes, which you can then read as 'utf8' text.

3️⃣ Joining Buffers & Streams

Because each buffer has a fixed size, you don't append to one — you combine several with Buffer.concat([a, b]) , which returns one new buffer containing all the bytes in order.

This is the exact pattern you use with streams : data arrives in pieces called chunks (each a Buffer), you push them into an array, and when the stream ends you Buffer.concat them into a single buffer to read as text or parse.

Your turn. Fill in the three blanks marked ___ , then run it and compare with the expected output.

No blanks this time — just a brief and an outline. Encoding a string to base64 and decoding it back is the foundation of countless real tasks (tokens, data URLs, basic auth headers). Write it yourself, run it, and check your output.

📋 Quick Reference — Buffers

Practice quiz

What is a Buffer in Node.js?

A resizable string
A network socket
A fixed-length block of raw bytes
A JSON object

Answer: A fixed-length block of raw bytes. A Buffer is a fixed-length chunk of raw bytes, used for files, network data, and binary content.

What does Buffer.from('Hello') create?

A buffer of the utf8 bytes for 'Hello'
The number 5
A base64 string
An empty buffer

Answer: A buffer of the utf8 bytes for 'Hello'. Buffer.from(string) encodes the text as bytes, utf8 by default, so 'Hello' becomes 5 bytes.

What is buf.length measured in?

Characters
Bits
Words
Bytes

Answer: Bytes. A buffer's length is the number of bytes, which may exceed the visible character count for multi-byte text.

What does Buffer.alloc(4) produce?

A buffer with random leftover memory
A zero-filled buffer of 4 bytes
A 4-character string
An error

Answer: A zero-filled buffer of 4 bytes. Buffer.alloc(n) creates an n-byte buffer that is zero-filled and safe to read immediately.

How does Buffer.alloc differ from Buffer.allocUnsafe?

alloc zero-fills the memory; allocUnsafe may contain leftover data
They are identical
allocUnsafe is always zero-filled
alloc is faster

Answer: alloc zero-fills the memory; allocUnsafe may contain leftover data. allocUnsafe skips zeroing for speed but may expose leftover memory; use alloc by default.

What does buf.toString('hex') return?

The base64 form
The original text
Each byte as two hex digits
The byte count

Answer: Each byte as two hex digits. .toString('hex') shows each byte as two hexadecimal digits, handy for debugging.

Is base64 a form of encryption?

Yes, it hides secrets
No, it is encoding that anyone can decode
Yes, but only with a key
No, it is compression

Answer: No, it is encoding that anyone can decode. base64 is encoding, not encryption. Anyone can decode it; it is for moving binary data through text channels.

How do you combine two buffers into one?

a + b
a.push(b)
Buffer.join(a, b)

Buffers have a fixed size, so you combine several with Buffer.concat, which takes an array of buffers.

What does Buffer.concat([Buffer.from('Hello, '), Buffer.from('world!')]) have for .length?

Answer: 13. 'Hello, ' is 7 bytes and 'world!' is 6 bytes, so the joined buffer is 13 bytes.

By default, what type is each chunk delivered by a readable stream?

A string
A number
A Buffer
An array

Answer: A Buffer. Stream chunks are Buffers by default; you collect them and Buffer.concat at the end unless you set an encoding.