Buffer

Why Buffer exists

JavaScript strings are stored internally as UTF-16 — two bytes per code point in the Basic Multilingual Plane. That is fine for text, but networking and filesystem work deals in raw bytes: a TCP packet, a JPEG image, or a binary protocol frame has no inherent encoding.

Buffer is Node’s answer: a fixed-length sequence of bytes exposed as a Uint8Array subclass. It predates typed arrays and is still the standard type you will see throughout Node’s streams, HTTP, crypto, and fs APIs.

🟢 Strings are UTF-16 in JS; Buffer handles raw bytes

A JavaScript string with 4 characters might occupy 4, 8, or even more bytes depending on the encoding. Buffer separates the concept of “how many characters” from “how many bytes on the wire.” Always use Buffer.byteLength(str, encoding) — not str.length — when you need the byte size of a string.

Creating a Buffer

The preferred way is Buffer.from(). The deprecated new Buffer() constructor was removed; never use it.

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// From a string (UTF-8 by default) var buf1 = Buffer.from('Hello'); console.log('buf1:', buf1); // <Buffer 48 65 6c 6c 6f> console.log('buf1 length:', buf1.length); // 5 bytes // From a hex string var buf2 = Buffer.from('deadbeef', 'hex'); console.log('buf2:', buf2); // <Buffer de ad be ef> console.log('buf2 length:', buf2.length); // 4 bytes // Allocate a zeroed buffer of N bytes var buf3 = Buffer.alloc(4); console.log('buf3 (zeroed):', buf3); // <Buffer 00 00 00 00>

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Encodings: utf8, hex, base64

toString(encoding) converts a Buffer back to a string in the requested encoding.

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var original = 'Node.js'; var buf = Buffer.from(original, 'utf8'); console.log('utf8 :', buf.toString('utf8')); console.log('hex :', buf.toString('hex')); console.log('base64:', buf.toString('base64')); // Round-trip: base64 -> Buffer -> string var b64 = buf.toString('base64'); var decoded = Buffer.from(b64, 'base64').toString('utf8'); console.log('decoded:', decoded);

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Byte length vs string length

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// ASCII: 1 char = 1 byte var ascii = 'hello'; console.log('ascii chars:', ascii.length); console.log('ascii bytes:', Buffer.byteLength(ascii, 'utf8')); // Thai: multi-byte in UTF-8 var thai = 'สวัสดี'; console.log('thai chars:', thai.length); console.log('thai bytes (utf8):', Buffer.byteLength(thai, 'utf8')); // Emoji: surrogate pair in JS = 2 .length units var emoji = 'Node 🚀'; console.log('emoji .length:', emoji.length); console.log('emoji bytes (utf8):', Buffer.byteLength(emoji, 'utf8'));

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Slicing and copying

buf.slice(start, end) (or buf.subarray) returns a view into the same memory — modifying the slice modifies the original. Use Buffer.from(slice) to get an independent copy.

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var buf = Buffer.from('Hello, World!'); var slice = buf.slice(0, 5); console.log('slice:', slice.toString()); // Hello // slice shares memory — mutating it mutates buf slice[0] = 0x4A; // 'J' console.log('buf after mutation:', buf.toString()); // Jello, World! // Independent copy var copy = Buffer.from(buf.slice(7, 12)); copy[0] = 0x57; // 'W' console.log('copy:', copy.toString()); // World with W console.log('buf unchanged:', buf.toString());

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Why is Buffer.byteLength(str) often different from str.length?

Buffer counts null terminators; strings do not str.length counts UTF-16 code units; Buffer.byteLength counts bytes in the chosen encoding They are always equal for ASCII Buffer.byteLength counts bits, not bytes

What does Buffer.from("deadbeef", "hex") produce?

A Buffer containing the ASCII characters d, e, a, d, b, e, e, f A Buffer of 4 bytes decoded from the hex string An error — hex is not a valid encoding for Buffer.from A Buffer of 8 bytes, one per hex character

What happens when you mutate a slice returned by buf.slice()?

The slice is an independent copy — the original is unaffected Both the slice and the original buffer are modified An error is thrown — Buffer slices are read-only Only the original is modified

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