It's a silly world out there.

derp-case.js

#!/usr/bin/env node

const fs = require('fs');
const readline = require('readline');

const rl = readline.createInterface({
	input: process.stdin,
	output: process.stdout,
	terminal: false,
});

rl.on('line', line => {
	console.log(derpCase(line));
});

function derpCase(string) {
	return Array.from(string).map(c =>
		Math.random() > 0.5
		? c.toUpperCase()
		: c.toLowerCase()
	).join('');
}

$ echo 'Hello, world!' | derp-case 
hELlo, wOrLd!