Use a Node.js worker thread with a FIFO to implement a service call that returns to the main thread synchronously.

blocking-the-main-thread.js

const {
  Worker,
  isMainThread,
  parentPort,
  MessageChannel,
  receiveMessageOnPort,
} = require('worker_threads');
const fs = require('fs');

const FIFO_NAME = '/tmp/wake-main-thread';

if (isMainThread) {
  console.log('starting main thread');
  const { execSync } = require('child_process');
  execSync(`rm -f ${FIFO_NAME}; mkfifo ${FIFO_NAME}`);

  let count = 0;
  setInterval(() => {
    if (count ++ >= 3) {
      process.exit(0);
    }
    console.log('counter is alive', count);
  }, 1000);

  const worker = new Worker(__filename);

  // We need to create a new MessageChannel because the default
  // worker port cannot be reified as an argument to receiveMessageOnPort;
  const ioChannel = new MessageChannel();
  worker.postMessage(ioChannel.port1, [ioChannel.port1]);

  console.log('sending request to worker');
  ioChannel.port2.postMessage({id: 1, arg: 'Hello, world!'});
  let reply = receiveMessageOnPort(ioChannel.port2);
  while (reply === undefined) {
    console.log('blocking main thread on', FIFO_NAME);
    fs.readFileSync(FIFO_NAME); // block
    console.log('waking main thread from', FIFO_NAME);
    reply = receiveMessageOnPort(ioChannel.port2);
  }
  console.log('main thread received', reply);
} else {
  parentPort.on('message', port => {
    // New IO instance.
    port.on('message', async request => {
      // Simulate some async work.
      await new Promise(resolve => setTimeout(resolve, 1500));
      const reply = {id: request.id, reply: `got: ${request.arg}`};

      // Return the message, and wake the main thread if it isn't already.
      port.postMessage(reply);
      fs.writeFileSync(FIFO_NAME, ''); // unblock
    });
  });
}

The output, under MacOS (I'd expect Linux would work too):

soil:cosmic-swingset michael$ node blocking-the-main-thread.js 
starting main thread
sending request to worker
blocking main thread on /tmp/wake-main-thread
waking main thread from /tmp/wake-main-thread
main thread received { message: { id: 1, reply: 'got: Hello, world!' } }
counter is alive 1
counter is alive 2
counter is alive 3
soil:cosmic-swingset michael$