prototype/src/form.js

var Form = {
  reset: function(form) {
    $(form).reset();
    return form;
  },
  
  serializeElements: function(elements, options) {
    if (typeof options != 'object') options = { hash: !!options };
    else if (Object.isUndefined(options.hash)) options.hash = true;
    var key, value, submitted = false, submit = options.submit;
    
    var data = elements.inject({ }, function(result, element) {
      if (!element.disabled && element.name) {
        key = element.name; value = $(element).getValue();
        if (value != null && (element.type != 'submit' || (!submitted &&
            submit !== false && (!submit || key == submit) && (submitted = true)))) { 
          if (key in result && !Object.isFunction(result[key])) {
            // a key is already present; construct an array of values
            if (!Object.isArray(result[key])) result[key] = [result[key]];
            result[key].push(value);
          }
          else result[key] = value;
        }
      }
      return result;
    });
    
    return options.hash ? data : Object.toQueryString(data);
  }
};

Form.Methods = {
  serialize: function(form, options) {
    return Form.serializeElements(Form.getElements(form), options);
  },
  
  getElements: function(form) {
    return $A($(form).getElementsByTagName('*')).inject([], 
      function(elements, child) {
        if (Form.Element.Serializers[child.tagName.toLowerCase()])
          elements.push(Element.extend(child));
        return elements;
      }
    );
  },
  
  getInputs: function(form, typeName, name) {
    form = $(form);
    var inputs = form.getElementsByTagName('input');
    
    if (!typeName && !name) return $A(inputs).map(Element.extend);
      
    for (var i = 0, matchingInputs = [], length = inputs.length; i < length; i++) {
      var input = inputs[i];
      if ((typeName && input.type != typeName) || (name && input.name != name))
        continue;
      matchingInputs.push(Element.extend(input));
    }

    return matchingInputs;
  },

  disable: function(form) {
    form = $(form);
    Form.getElements(form).invoke('disable');
    return form;
  },

  enable: function(form) {
    form = $(form);
    Form.getElements(form).invoke('enable');
    return form;
  },

  findFirstElement: function(form) {
    var elements = $(form).getElements().findAll(function(element) {
      return 'hidden' != element.type && !element.disabled;
    });
    var firstByIndex = elements.findAll(function(element) {
      return element.hasAttribute('tabIndex') && element.tabIndex >= 0;
    }).sortBy(function(element) { return element.tabIndex }).first();
    
    return firstByIndex ? firstByIndex : elements.find(function(element) {
      return ['input', 'select', 'textarea'].include(element.tagName.toLowerCase());
    });
  },

  focusFirstElement: function(form) {
    form = $(form);
    form.findFirstElement().activate();
    return form;
  },
  
  request: function(form, options) {
    form = $(form), options = Object.clone(options || { });

    var params = options.parameters, action = form.readAttribute('action') || '';
    if (action.blank()) action = window.location.href;
    options.parameters = form.serialize(true);
    
    if (params) {
      if (Object.isString(params)) params = params.toQueryParams();
      Object.extend(options.parameters, params);
    }
    
    if (form.hasAttribute('method') && !options.method)
      options.method = form.method;
    
    return new Ajax.Request(action, options);
  }
};

/*--------------------------------------------------------------------------*/

Form.Element = {
  focus: function(element) {
    $(element).focus();
    return element;
  },

  select: function(element) {
    $(element).select();
    return element;
  }
};

Form.Element.Methods = {
  serialize: function(element) {
    element = $(element);
    if (!element.disabled && element.name) {
      var value = element.getValue();
      if (value != undefined) {
        var pair = { };
        pair[element.name] = value;
        return Object.toQueryString(pair);
      }
    }
    return '';
  },
  
  getValue: function(element) {
    element = $(element);
    var method = element.tagName.toLowerCase();
    return Form.Element.Serializers[method](element);
  },

  setValue: function(element, value) {
    element = $(element);
    var method = element.tagName.toLowerCase();
    Form.Element.Serializers[method](element, value);
    return element;
  },

  clear: function(element) {
    $(element).value = '';
    return element;
  },

  present: function(element) {
    return $(element).value != '';
  },
  
  activate: function(element) {
    element = $(element);
    try {
      element.focus();
      if (element.select && (element.tagName.toLowerCase() != 'input' ||
          !['button', 'reset', 'submit'].include(element.type)))
        element.select();
    } catch (e) { }
    return element;
  },
  
  disable: function(element) {
    element = $(element);
    element.blur();
    element.disabled = true;
    return element;
  },
  
  enable: function(element) {
    element = $(element);
    element.disabled = false;
    return element;
  }
};

/*--------------------------------------------------------------------------*/

var Field = Form.Element;
var $F = Form.Element.Methods.getValue;

/*--------------------------------------------------------------------------*/

Form.Element.Serializers = {
  input: function(element, value) {
    switch (element.type.toLowerCase()) {
      case 'checkbox':  
      case 'radio':
        return Form.Element.Serializers.inputSelector(element, value);
      default:
        return Form.Element.Serializers.textarea(element, value);
    }
  },

  inputSelector: function(element, value) {
    if (Object.isUndefined(value)) return element.checked ? element.value : null;
    else element.checked = !!value;
  },

  textarea: function(element, value) {
    if (Object.isUndefined(value)) return element.value;
    else element.value = value;
  },
  
  select: function(element, index) {
    if (Object.isUndefined(index))
      return this[element.type == 'select-one' ? 
        'selectOne' : 'selectMany'](element);
    else {
      var opt, value, single = !Object.isArray(index);
      for (var i = 0, length = element.length; i < length; i++) {
        opt = element.options[i];
        value = this.optionValue(opt);
        if (single) {
          if (value == index) {
            opt.selected = true;
            return;
          }
        }
        else opt.selected = index.include(value);
      }
    }
  },
  
  selectOne: function(element) {
    var index = element.selectedIndex;
    return index >= 0 ? this.optionValue(element.options[index]) : null;
  },
  
  selectMany: function(element) {
    var values, length = element.length;
    if (!length) return null;
    
    for (var i = 0, values = []; i < length; i++) {
      var opt = element.options[i];
      if (opt.selected) values.push(this.optionValue(opt));
    }
    return values;
  },
  
  optionValue: function(opt) {
    // extend element because hasAttribute may not be native
    return Element.extend(opt).hasAttribute('value') ? opt.value : opt.text;
  }
};

/*--------------------------------------------------------------------------*/

Abstract.TimedObserver = Class.create(PeriodicalExecuter, {
  initialize: function($super, element, frequency, callback) {
    $super(callback, frequency);
    this.element   = $(element);
    this.lastValue = this.getValue();
  },
  
  execute: function() {
    var value = this.getValue();
    if (Object.isString(this.lastValue) && Object.isString(value) ?
        this.lastValue != value : String(this.lastValue) != String(value)) {
      this.callback(this.element, value);
      this.lastValue = value;
    }
  }
});

Form.Element.Observer = Class.create(Abstract.TimedObserver, {
  getValue: function() {
    return Form.Element.getValue(this.element);
  }
});

Form.Observer = Class.create(Abstract.TimedObserver, {
  getValue: function() {
    return Form.serialize(this.element);
  }
});

/*--------------------------------------------------------------------------*/

Abstract.EventObserver = Class.create({
  initialize: function(element, callback) {
    this.element  = $(element);
    this.callback = callback;
    
    this.lastValue = this.getValue();
    if (this.element.tagName.toLowerCase() == 'form')
      this.registerFormCallbacks();
    else
      this.registerCallback(this.element);
  },
  
  onElementEvent: function() {
    var value = this.getValue();
    if (this.lastValue != value) {
      this.callback(this.element, value);
      this.lastValue = value;
    }
  },
  
  registerFormCallbacks: function() {
    Form.getElements(this.element).each(this.registerCallback, this);
  },
  
  registerCallback: function(element) {
    if (element.type) {
      switch (element.type.toLowerCase()) {
        case 'checkbox':  
        case 'radio':
          Event.observe(element, 'click', this.onElementEvent.bind(this));
          break;
        default:
          Event.observe(element, 'change', this.onElementEvent.bind(this));
          break;
      }
    }    
  }
});

Form.Element.EventObserver = Class.create(Abstract.EventObserver, {
  getValue: function() {
    return Form.Element.getValue(this.element);
  }
});

Form.EventObserver = Class.create(Abstract.EventObserver, {
  getValue: function() {
    return Form.serialize(this.element);
  }
});