If your application really requires long-running JavaScript code, one of the best ways to deal with it is by using JavaScript web workers. JavaScript code normally runs on the foreground thread, but by creating a web worker you can effectively keep a long-running process on a background thread, and your UI thread will be free to respond to user input.

As an example, you create a new worker like this:

var myWorker = new Worker(worker.js);

You can then post messages to it from the js in the main page like this:

myWorker.postMessage([first.value,second.value]);

console.log('Message posted to worker');

And respond to the messages in worker.js like this:

onmessage = function(e) {

  console.log('Message received from main script');

  var workerResult = 'Result: ' + (e.data[0] * e.data[1]);

  console.log('Posting message back to main script');

  postMessage(workerResult);

}