This script starts a barebones Node.js server that has three APIs.

sync_seriousness_demo.js

'use strict';

const http = require('http');
const childProcess = require('child_process');

const server = http.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');

  if (req.url === '/quick-api') {
    return res.end('<h1>This API must return immediately.</h1>');
  }
  if (req.url === '/slow-sync-api') {
    childProcess.execSync('sleep 30');
    return res.end('<h1>This API performs a slow synchronous operation. '
      + 'It will block all other requests '
      + 'and bring down the performance of the entire application.</h1>');
  }
  if (req.url === '/slow-async-api') {
    return childProcess.exec('sleep 30', () => {
      return res.end('<h1>This API also performs a slow operation. '
      + 'But it does not block other requests. '
      + 'The performance of rest of the application is unaffected by such slow APIs</h1>');
    })
  }
})

server.listen(3000, () => {
  console.log(`Server is listening`);
});

To run this script, just run node index.js. There are no additional packages to be installed.

This script starts a barebones Node.js server that has three APIs.

• GET /quick-api
• GET /slow-sync-api
• GET /slow-async-api

The quick-api API is supposed to return a response immediately.
But, if you call quick-api after slow-sync-api, it does not return until slow-sync-api returns. This is because Node.js is single-threaded. And slow-sync-api calls a synchronous method that does not let the main thread do anything else till its execution is complete. It blocks all other requests.
However, if you call quick-api after slow-async-api, the quick-api works as expected. This is because slow-async-api does not block the main thread till its execution is complete. The work of the asynchronous function is deferred to a worker thread and the main thread waits for the execution to complete. Meanwhile, the main thread is free to fulfill other requests.