map(enumerable, mapFunction, callback, numWorkers);

function map(data, mapper, callback, numWorkers) {
 
  // Support arrays & objects
  var length = 0;
  for(var d in data) { length++; }
  var result = new data.constructor;
 
  numWorkers = Math.min(numWorkers || 32, length);
  var workers = [];
  var messagesReceived = 0;
 
  // Create the workers
  for (var i=0; i < numWorkers; i++) {
    workers[i] = new Worker("mapper.js");
    workers[i].addEventListener('message', function(e) {
      result[e.data.key] = e.data.value;
      // Check if we have finished the job.  This should probably be more robust.
      if (++messagesReceived == length) { callback(result) };
    }, false);
  }
 
  // Just send out all the tasks.  The messages get queued by the browser.
  // It would probably be better to queue up two or three tasks per worker (to minimize downtime)
  // and add tasks to the queues as results come back.
  var nextItem=0
  for (var d in data) {
    workers[nextItem++ % numWorkers].postMessage({key: d, value: data[d], mapper: "(" + String(mapper) + ")(value)"});
  }
 
}

// Minion
onmessage = function(e) {
    var value = e.data.value;
    postMessage({key: e.data.key, value: eval(e.data.mapper)});
}