Allow JS to yield and allow GUI to update

Not implicitly. You can explicitly break your operation into multiple slices, and use the Task object to periodically call your worker function in slices. e.g. something like the following.

var current_slice=0;
var worker_task = new Task(doslice);

function startwork()
{
  current_slice=0;
  doslice();
}

function doslice()
{
  // do some portion of the work here;
  current_slice++;
  if (current_slice