slowdep

One import. Any async function.

Drop slowdep anywhere you call an external dependency. It preserves the original function signature — same inputs, same outputs, same errors. Just slower and more realistic.

import { withLatency } from 'slowdep';

// your real function, unchanged
const findUser = async (id) => db.query('SELECT * FROM users WHERE id = $1', [id]);

// wrap it — now behaves like real Postgres
const slowFindUser = withLatency(findUser, 'postgres');

const user = await slowFindUser(42); // every call is different

import { withLatency } from 'slowdep';

const slowFetch = withLatency(fetchExternalAPI, {
  p50: 100,         // typical call: ~100ms
  p99: 2000,        // worst 1%: up to 2 seconds
  errorRate: 0.01,  // 1% throw a transient error
});

const data = await slowFetch('/api/users');

import { withLatencyAll } from 'slowdep';

// wrap every method on a client at once
const slowRedis = withLatencyAll(redisClient, 'redis');

await slowRedis.get('key');      // slow
await slowRedis.set('k', 'v');  // slow
await slowRedis.del('key');     // slow

Real-world latency profiles, built in.

Based on real p50/p95/p99 data from production systems. Pass a string and you're done.

Why setTimeout is lying to you.

A flat delay gives every call the same wait time. Real dependencies don't work like that — they respond in 5ms most of the time, hit 200ms occasionally, and spike past 1s when GC pauses, cold caches, or noisy neighbors show up.

slowdep fits a lognormal distribution to your p50 and p99 values using the Box-Muller transform. The result: fast most of the time, with a realistic long tail — the exact shape that breaks retry logic, timeouts, and circuit breakers in production.