gistfile1.js

var BackgroundIteration = Class.create({
  initialize: function(iterator, produceNextValue) {
    this.iterator = iterator;
    this.produceNextValue = produceNextValue;
    this.callbacks = [];
    this.results = [];

    this.next = this.next.bind(this);
    this.next.defer();
  },
  
  next: function() {
    var value = {}, hasValue = this.produceNextValue(value);
    this.running = true;

    if (hasValue) {
      try {
        this.results.push(this.iterator(value.value));
        this.next.defer();
      } catch (exception) {
        this.exception = exception;
        this.end();
      }
    } else {
      this.end();
    }
  },
  
  end: function() {
    if (this.running) {
      this.running = false;
      this.callbacks.invoke("call", this);
    }
  },
  
  after: function(callback) {
    this.callbacks.push(callback);
    return this;
  }
});

Object.extend(Array.prototype, {
  backgroundEach: function(iterator) {
    var index = 0;
    return new BackgroundIteration(iterator, function(memo) {
      if (index == this.length) {
        return false;
      } else {
        memo.value = this[index++];
        return true;
      }
    }.bind(this));
  }
});