Use WebWorkers and promises to run sync heavy functions in a worker (process) and get the result in a promise

async-thread.js

function asyncThread(fn, ...args) {
  if (!window.Worker) throw Promise.reject(
    new ReferenceError(`WebWorkers aren't available.`)
  );
  
  const fnWorker = `
  self.onmessage = function(message) {
    (${fn.toString()})
      .apply(null, message.data)
      .then(result => self.postMessage(result));
  }`;
  
  return new Promise((resolve, reject) => {
    try {
      const blob = new Blob([fnWorker], { type: 'text/javascript' });
      const blobUrl = window.URL.createObjectURL(blob);
      const worker = new Worker(blobUrl);
      window.URL.revokeObjectURL(blobUrl);
      
      worker.onmessage = result => {
        resolve(result.data);
        worker.terminate();
      };

      worker.onerror = error => {
        reject(error);
        worker.terminate();
      };

      worker.postMessage(args);
    } catch (error) {
      reject(error);
    }
  });
}

thread.js

function thread(fn, ...args) {
  if (!window.Worker) throw Promise.reject(
    new ReferenceError(`WebWorkers aren't available.`)
  );

  const fnWorker = `
  self.onmessage = function(message) {
    self.postMessage(
      (${fn.toString()}).apply(null, message.data)
    );
  }`;

  return new Promise((resolve, reject) => {
    try {
      const blob = new Blob([fnWorker], { type: 'text/javascript' });
      const blobUrl = window.URL.createObjectURL(blob);
      const worker = new Worker(blobUrl);
      window.URL.revokeObjectURL(blobUrl);
      
      worker.onmessage = result => {
        resolve(result.data);
        worker.terminate();
      };

      worker.onerror = error => {
        reject(error);
        worker.terminate();
      };

      worker.postMessage(args);
    } catch (error) {
      reject(error);
    }
  });
}

// example with an async/await function
(async () => {
  try {
    const res1 = await thread(
      str => JSON.parse(str),
      '{"key": "value"}'
    );
    console.log(res1);
  } catch (error) {
    console.error(error);
  }
})();

// example using a sum function and promise syntax
// function sum(n1, n2) {
//   return n1 + n2;
// }
// thread(sum, 1, 5)
//   .then(result => thread(sum, result, 1))
//   .then(result => console.log('result', result))
//   .catch(error => console.error('error', error));