What is the Node.js thread pool?

Direct answer

libuv maintains a fixed-size thread pool (default 4 threads, UV_THREADPOOL_SIZE) for work that cannot use fully non-blocking OS APIs — including fs.* on some systems, crypto, compression, and dns.lookup.

The main JavaScript thread stays free, but pool threads do blocking work on Node's behalf. If all four are busy, fifth crypto.hash waits in queue — latency spikes under load.

Often uses thread pool	Usually true async (no pool)
fs.readFile / write on some platforms	TCP net.Server accept/read
crypto.pbkdf2, scrypt, randomBytes (some paths)	net.connect on modern OS
dns.lookup (not dns.resolve)	fs.read on Linux io_uring paths (evolving)
zlib compression	Most timer and setImmediate scheduling

UV_THREADPOOL_SIZE — set env var before process start; max 1024; tune for crypto-heavy APIs.
dns.lookup vs dns.resolve — resolve uses network (no pool); lookup uses getaddrinfo (pool) — common perf interview detail.
Not unlimited parallelism — more concurrent bcrypt than pool size queues — use worker_threads for isolation (Q69).

thread-pool-crypto.js

import { pbkdf2 } from "node:crypto";

console.log("Starting 6 password hashes on 4 pool threads…");

for (let i = 0; i < 6; i++) {
  pbkdf2("password", "salt", 100000, 64, "sha512", () => {
    console.log(`hash ${i} done`);
  });
}

// First 4 run in parallel; 5th and 6th wait for a free thread