Sys.UI.Behavior

            Type.registerNamespace('Samples');
            Samples.EmptyBehavior = function(element) {
            Samples.EmptyBehavior.initializeBase(this, [element]);
            }
            Samples.EmptyBehavior.prototype = {
            initialize : function() {
            Samples.EmptyBehavior.callBaseMethod(this, 'initialize');
            },
            dispose : function() {
            Samples.EmptyBehavior.callBaseMethod(this, 'dispose');
            }
            }
            Samples.EmptyBehavior.registerClass('Samples.EmptyBehavior',
            Sys.UI.Behavior);

            //Creating behavior

            $create(Samples.EmptyBehavior, {'name':'myEmptyBehavior'}, {}, {},
            $get('elementID'));

            //accessing behaviors
            var instance = $find('someElement$myEmptyBehavior');
            //or
            var emptyBehaviorInstance = $get('someElement').myEmptyBehavior;
            

        <pre data-sub="prettyprint:_">
        Type.registerNamespace('Samples');

        // Samples.FormattingBehavior class.
        Samples.FormattingBehavior = function(element) {
        Samples.FormattingBehavior.initializeBase(this, [element]);

        // Fields.
        this._hoverCssClass = null;
        this._focusCssClass = null;
        this._currentCssClass = null;
        this._mouseOver = null;
        this._focus = null;
        }
        Samples.FormattingBehavior.prototype = {
        // Component lifecycle.
        initialize : function() {
        Samples.FormattingBehavior.callBaseMethod(this, 'initialize');
        //this is changing the
        //the behavior of the associated
        //element
        //get_element is inherit from parent.

        $addHandlers(this.get_element(),
        {
        mouseout:this._onMouseout,
        mouseover:this._onMouseover,
        focus:this._onFocus,
        blur:this._onBlur
        }, this);
        },

        dispose : function() {
        $clearHandlers(this.get_element());

        Samples.FormattingBehavior.callBaseMethod(this, 'dispose');
        },

        // Event handlers.
        _onMouseover : function() {
        this._mouseOver = true;
        this._setCssClass();
        },

        _onMouseout : function() {
        this._mouseOver = false;
        this._setCssClass();
        },

        _onFocus : function() {
        this._focus = true;
        this._setCssClass();
        },

        _onBlur : function() {
        this._focus = false;
        this._setCssClass();
        },

        // Helper method.
        _setCssClass : function() {
        if (this._currentCssClass) {
        Sys.UI.DomElement.removeCssClass(this._element,
        this._currentCssClass);
        this._currentCssClass = null;
        }

        if (this._error) {
        this._currentCssClass = this._errorCssClass;
        }
        else if (this._focus) {
        this._currentCssClass = this._focusCssClass;
        }
        else if (this._mouseOver) {
        this._currentCssClass = this._hoverCssClass;
        }

        if (this._currentCssClass) {
        Sys.UI.DomElement.addCssClass(this._element,
        this._currentCssClass);
        }
        },

        // Properties.
        get_hoverCssClass : function() {
        return this._hoverCssClass;
        },

        set_hoverCssClass : function(value) {
        this._hoverCssClass = value;
        },

        get_focusCssClass : function() {
        return this._focusCssClass;
        },

        set_focusCssClass : function(value) {
        this._focusCssClass = value;
        }
        }
        // Inherit from Sys.UI.Behavior.
        Samples.FormattingBehavior.registerClass('Samples.FormattingBehavior', Sys.UI.Behavior);


        //add the client side we attached the behavior to element
        Sys.Application.add_init(pageInit);

        function pageInit(sender, e) {
        // Attach the FormattingBehavior to the txtName textbox.
        $create(Samples.FormattingBehavior,
        {'hoverCssClass':'field_hover', 'focusCssClass':'field_focus'},
        {}, {}, $get('Name'));

        // Attach the FormattingBehavior to the txtLastName textbox.
        $create(Samples.FormattingBehavior,
        {'hoverCssClass':'field_hover', 'focusCssClass':'field_focus'},
        {}, {}, $get('LastName'));
        }
        //$create function will automatically call the initialize method.
        </pre>