9. The fs module

`fs` reads and writes the filesystem in three flavours: synchronous (`readFileSync`, blocks the event loop), callback-based (`readFile(path, cb)`), and promise-based via `fs/promises`.

js
import { readFile, writeFile } from "node:fs/promises";

const config = JSON.parse(await readFile("./config.json", "utf8"));
await writeFile("./out.json", JSON.stringify({ ok: true }, null, 2));

Prefer the async APIs on servers: a synchronous read blocks every other request being handled on that same event loop. Sync APIs are fine for one-off scripts and startup-time config loading, where blocking briefly at boot is harmless.

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