JavaScript: Run a function in a separate thread by using a Web Worker, allowing long running functions to not block the UI

const runAsync = fn => {
  const worker = new Worker(
    URL.createObjectURL(new Blob([`postMessage((${fn})());`]), {
      type: 'application/javascript; charset=utf-8'
    })
  );
  return new Promise((res, rej) => {
    worker.onmessage = ({ data }) => {
      res(data), worker.terminate();
    };
    worker.onerror = err => {
      rej(err), worker.terminate();
    };
  });
};
const longRunningFunction = () => {
  let result = 0;
  for (let i = 0; i < 1000; i++) {
    for (let j = 0; j < 700; j++) {
      for (let k = 0; k < 300; k++) {
        result = result + i + j + k;
      }
    }
  }
  return result;
};
/*
*/
runAsync(longRunningFunction).then(console.log); // 209685000000
runAsync(() => 10 ** 3).then(console.log); // 1000
let outsideVariable = 50;
runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'

Output:

1000
undefined