The source code

    /**
     * @class Ext.form.Hidden
     * @extends Ext.form.Field
     * A basic hidden field for storing hidden values in forms that need to be passed in the form submit.
     * @constructor
     * Create a new Hidden field.
     * @param {Object} config Configuration options
     * @xtype hidden
     */
    Ext.form.Hidden = Ext.extend(Ext.form.Field, {
        // private
        inputType : 'hidden',

        shouldLayout: false,

        // private
        onRender : function(){
            Ext.form.Hidden.superclass.onRender.apply(this, arguments);
        },

        // private
        initEvents : function(){
            this.originalValue = this.getValue();
        },

        // These are all private overrides
        setSize : Ext.emptyFn,
        setWidth : Ext.emptyFn,
        setHeight : Ext.emptyFn,
        setPosition : Ext.emptyFn,
        setPagePosition : Ext.emptyFn,
        markInvalid : Ext.emptyFn,
        clearInvalid : Ext.emptyFn
    });
    Ext.reg('hidden', Ext.form.Hidden);