How do Readable and Writable streams work in Node.js?

Direct answer

Readable emits 'data' chunks or is consumed via async iteration/read(); Writable accepts write() chunks and signals completion with end() — both pause when internal buffers exceed highWaterMark until the consumer catches up.

Readable streams produce data; Writable streams consume it. Modes matter: flowing mode auto-emits 'data'; paused mode uses read() explicitly.

Readable	Writable
on('data', fn) / for await	write(chunk, cb)
on('end') when exhausted	end() / end(chunk) to finish
pause() / resume()	cork() / uncork() batch writes
pipe(writable)	Returns backpressure signal via write return false

Create readable — fs.createReadStream or Readable.from(array).
Consume — pipeline to writable or collect with async iteration.
Handle errors — 'error' event on both sides; destroy stream on failure.

readable-writable.mjs

import fs from "node:fs";

const readable = fs.createReadStream("large-export.ndjson", { encoding: "utf8" });
const writable = fs.createWriteStream("filtered.ndjson");

readable.on("data", (line) => {
  if (line.includes("nodejs")) writable.write(line);
});

readable.on("end", () => writable.end());
readable.on("error", (err) => {
  writable.destroy(err);
});

// Better: await pipeline(readable, transform, writable)