Another simple implementation of pushing a potentially costly operation off to a worker thread

inlineFuncAsWorker.js

//main function for creating an inline worker:
//inlineWorker:: Function -> a -> Promise b
const inlineWorker = fn => msg =>  {
  const scriptString = `const func = ${fn.toString()}; 
    addEventListener('message', function(e) {
      Promise.resolve(e.data)
        .then(func)
        .then(postMessage);
    }, false);
  `;
  
  //promisify the response/error for make things easier to consume
  const promise = new Promise( (resolve, reject) => {
    const blob = new Blob([scriptString], { type: "text/javascript" });
    const bloblurl = window.URL.createObjectURL(blob);
    const worker = new Worker(bloblurl);

    worker.onmessage = e => {
      resolve(e.data);
      window.URL.revokeObjectURL(bloblurl);//remember to free up the blob url as well
      worker.terminate();
    };
    worker.onerror = e => {
      reject(e);
      window.URL.revokeObjectURL(bloblurl);
      worker.terminate();
    };

    worker.postMessage(msg);
  });

  return promise;
}

//examples

//one line usage
inlineWorker(x=>x+1)(6).then(x=>console.log('msg returned: ',x));//logs "7"

//or elevate functions for repeated usage
const addOneWW = inlineWorker(x=>x+1);
//returns a promise-returning function that will take 
//a value and run the function on it... just in a webworker