What are the types of streams in Node.js?

Direct answer

Node has four stream types — Readable (source), Writable (destination), Duplex (both, e.g. TCP socket), and Transform (Duplex that modifies data, e.g. gzip); all extend EventEmitter and implement the stream API.

Classifying streams by data direction helps pick the right API and debug flow — interviewers often ask which type fits a file read vs HTTP proxy vs encryption.

Type	Direction	Examples
Readable	Read only	fs.createReadStream, http.IncomingMessage
Writable	Write only	fs.createWriteStream, http.ServerResponse
Duplex	Read + write independent	net.Socket, TLS socket
Transform	Read → transform → write	zlib.createGzip, crypto.createCipheriv

Object mode — streams push JS objects instead of Buffers/strings when objectMode: true.
Legacy vs web — Node stream API; also stream/web ReadableStream in fetch.
HighWaterMark — internal buffer threshold before backpressure kicks in (Q53).

stream-types.mjs

import { Readable, Writable, Transform } from "node:stream";

const source = Readable.from(["line1\n", "line2\n"]);
const upper = new Transform({
  transform(chunk, enc, cb) {
    cb(null, chunk.toString().toUpperCase());
  },
});
const sink = new Writable({
  write(chunk, enc, cb) {
    process.stdout.write(chunk);
    cb();
  },
});

source.pipe(upper).pipe(sink);