What is a Buffer in Node.js?

Direct answer

Buffer is Node's fixed-length raw binary data type — like Uint8Array for bytes from files, sockets, and crypto; use Buffer.from/alloc, avoid deprecated constructors, and specify encoding (utf8, hex, base64) when converting to strings.

JavaScript strings are UTF-16; Buffers hold bytes for I/O that is not text — images, hashes, TLS, file chunks in streams. Before TypedArrays were universal, Buffer was the Node binary workhorse.

Why interviewers ask

API	Purpose
Buffer.from(str, 'utf8')	String → bytes
buf.toString('hex')	Bytes → hex display
Buffer.alloc(n)	Zero-filled n-byte buffer
Buffer.concat(arr)	Join buffer chunks from stream
buf.length	Byte size, not character count

Stream chunks — default 'data' events emit Buffer unless encoding set on readable.
Uint8Array interop — modern APIs accept both; Buffer is Uint8Array subclass.
Security — zero fill on alloc; wipe sensitive buffers when done if handling secrets.

buffer-basics.mjs

const text = "AceDevHub";
const buf = Buffer.from(text, "utf8");

console.log(buf.length);        // bytes
console.log(buf.toString("hex"));

const chunks = [Buffer.from("hel"), Buffer.from("lo")];
const joined = Buffer.concat(chunks).toString("utf8"); // "hello"