Buffer

JavaScript · Reference cheat sheet

📋 Overview

In Node.js, Buffer is a fixed-length sequence of bytes (Uint8Array subclass). Use it for binary I/O, encodings, and crypto. Prefer Buffer.from / Buffer.alloc over the deprecated new Buffer().

import { Buffer } from "node:buffer";

🔧 Core concepts

• Buffer.from(data, encoding?): from string, array, ArrayBuffer, or another Buffer.
• Buffer.alloc(size, fill?, encoding?): zero-filled (safe) allocation.
• Buffer.allocUnsafe(size): faster, may contain old memory — overwrite before use.
• Encodings: 'utf8' (default for many APIs), 'utf16le', 'base64', 'base64url', 'hex', 'ascii', 'latin1'.
• buf.toString(encoding?): bytes → string.
• buf.length: byte length, not string character count for multi-byte UTF-8.
• Slice vs subarray: subarray views the same memory; mutating affects the parent.

💡 Examples

const a = Buffer.from("hello", "utf8");
const b = Buffer.from([0x68, 0x69]); // 'hi'
const z = Buffer.alloc(8); // 8 zero bytes

console.log(a.toString("utf8")); // 'hello'
console.log(a.toString("hex")); // '68656c6c6f'
console.log(Buffer.from("6869", "hex").toString()); // 'hi'

const b64 = Buffer.from("hi").toString("base64");
Buffer.from(b64, "base64").toString(); // 'hi'

// Concat
const all = Buffer.concat([a, b]);

// Compare / copy
Buffer.compare(a, b);
a.copy(Buffer.alloc(a.length));

// Binary file
import { readFile } from "node:fs/promises";
const buf = await readFile("image.png"); // Buffer

// UTF-8 byte length ≠ string length
const s = "é";
[...s].length; // 1
Buffer.byteLength(s, "utf8"); // 2

⚠️ Pitfalls

• allocUnsafe can leak old data if not filled — prefer alloc unless profiling demands otherwise.
• Truncating multi-byte UTF-8 mid-character corrupts strings — decode carefully when slicing.
• buf.slice is deprecated legacy alias; use subarray and remember shared memory.
• Mixing encodings (e.g. treat base64 as utf8) produces garbage.
• Large buffers: stream instead of loading entire files when possible.

🔗 Related

• fs — readFile returns Buffer by default
• Typed arrays — Uint8Array / ArrayBuffer
• Crypto — hashes over Buffers
• Stream — chunked Buffers
• encode — URL/text encoding related concepts