worker.js

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

if (isMainThread) {
    console.log('Starting the main thread');

    const worker = new Worker(__filename);

    worker.on('message', (msg) => {
        console.log(`Worker: ${msg}`);
    });

    console.log('Still in main thread');
} else {
    parentPort.postMessage('Getting started');

    wasteTime();

    parentPort.postMessage('In the middle');

    wasteTime();

    parentPort.postMessage('All done');

}

function wasteTime() {
    const end = Date.now() + 3000;
    while (Date.now() < end) { }
}