Streams

Streams process data in chunks instead of loading entire payloads into memory. Node provides four base types:

| Type | Role | |------|------| | Readable | Source — `read()`, `'data'`, `'end'` | | Writable | Sink — `write()`, `end()`, drain events | | Duplex | Independent read/write (e.g. TCP socket) | | Transform | Duplex that modifies data (gzip, JSON lines) |

Prefer `stream/promises.pipeline` for automatic error propagation and cleanup:

					import { createReadStream } from 'node:fs';
import { createGzip } from 'node:zlib';
import { pipeline } from 'node:stream/promises';

await pipeline(
  createReadStream('access.log'),
  createGzip(),
  createWriteStream('access.log.gz'),
);
				

Object mode streams carry JavaScript objects instead of Buffers/strings — useful for ETL, dangerous if producers outpace consumers.

On interviews: explain why streaming beats `readFile` for large files, how backpressure signals slow producers, and how pipeline differs from manual `.pipe()`.

Common pitfalls: listening to `'data'` without pausing and overwhelming memory; forgetting error handlers on piped streams; mixing callback and promise APIs without awaiting pipeline.

The trade-off is balancing simplicity, performance, safety, and operability — name which axis you optimized and what cost you accepted.

Checklist:

• Name four stream types and a use case each.
• Use `pipeline` for error-safe composition.
• Handle `'error'` on every leg.
• Respect `write()` return value and `'drain'`.