sl0-mo

A lightweight Node.js library to add artificial latency and random errors to async functions and Express routes. Ideal for testing how your application handles slow networks or flaky services.

Installation

npm install sl0-mo

Usage

Wrapper for Async Functions

import { withLatency } from 'sl0-mo';

// Your original function
const fetchData = async (id: number) => {
  // ... fetch data
  return { id, name: "Item" };
};

// Wrapped version
const delayedFetch = withLatency(fetchData, {
  min: 200,      // Minimum latency in ms (default: 200)
  max: 1000,     // Maximum latency in ms (default: 800)
  errorRate: 0.1 // 10% chance of throwing an error (default: 0)
});

// Use it just like the original
try {
  const data = await delayedFetch(123);
  console.log(data);
} catch (err) {
  console.error("Request failed!");
}

Express Middleware

import express from 'express';
import { latencyMiddleware } from 'sl0-mo';

const app = express();

// Add latency to all routes
app.use(latencyMiddleware({
  min: 500,
  max: 1500,
  errorRate: 0.05
}));

app.get('/', (req, res) => {
  res.send('Delayed response');
});

app.listen(3000);

API

withLatency(fn, options)

Wraps an async function.

• fn: The async function to wrap.
• options:
  • min (number): Minimum delay in ms.
  • max (number): Maximum delay in ms.
  • errorRate (number): Probability of error (0-1).

latencyMiddleware(options)

Express middleware.

• options: Same as withLatency.

License

MIT