Senior backend interviews
don't test syntax.
They test why your system crashes under 10k QPS.

// Contrasting Synchronous CPU-Blocking vs Non-Blocking I/O in Node.js
import http from 'node:http';
import fs from 'node:fs/promises';

const server = http.createServer(async (req, res) => {
  if (req.url === '/non-blocking') {
    // Non-blocking asynchronous I/O: Event loop yields while waiting for OS
    const data = await fs.readFile('./package.json', 'utf-8');
    res.writeHead(200, { 'Content-Type': 'application/json' });
    return res.end(data);
  }

  if (req.url === '/blocking-cpu-danger') {
    // ANTI-PATTERN: Synchronous CPU execution locks the single main thread!
    // During this loop, zero other incoming requests can be accepted or answered.
    const start = Date.now();
    let count = 0;
    for (let i = 0; i < 1e8; i++) {
      count += i;
    }
    const duration = Date.now() - start;
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    return res.end(`Stalled the event loop for ${duration}ms! Result: ${count}`);
  }

  res.writeHead(404);
  res.end('Not Found');
});

server.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});