How to get form field value in onclick event

I am using this article of architecture http://blog.extjs.eu/know-how/writing-a-big-application-in-ext/

in my code:

I have this Application.DashBoardForm.js in this i want to pass the value of the fromdate in the onclick event function , how can i pass the fromdate value ?

Ext.apply(Ext.form.VTypes, {
    daterange : function(val, field) {
        var date = field.parseDate(val);

        if(!date){
            return false;
        }
        if (field.startDateField) {
            var start = Ext.getCmp(field.startDateField);
            if (!start.maxValue || (date.getTime() != start.maxValue.getTime())) {
                start.setMaxValue(date);
                start.validate();
            }
        }
        else if (field.endDateField) {
            var end = Ext.getCmp(field.endDateField);
            if (!end.minValue || (date.getTime() != end.minValue.getTime())) {
                end.setMinValue(date);
                end.validate();
            }
        }
        /*
         * Always return true since we're only using this vtype to set the
         * min/max allowed values (these are tested for after the vtype test)
         */
        return true;
    }
});

Application.DashBoardForm= Ext.extend(Ext.FormPanel, {
     border:false
    ,initComponent:function() {
        var config = {
            labelWidth: 125,
            frame: true,
            title: 'Date Range',
            bodyStyle:'padding:5px 5px 0',
            width: 350,
            defaults: {width: 175},
            defaultType: 'datefield',
            items: [{
                fieldLabel: 'Start Date',
                name: 'fromdate',
                id: 'fromdate',
                vtype: 'daterange',
                value : new Date(),
                endDateField: 'todate' // id of the end date field
            },{
                fieldLabel: 'End Date',
                name: 'todate',
                id: 'todate',
                vtype: 'daterange',
                value : new Date(),
                startDateField: 'fromdate' // id of the start date field
            }]
            ,buttons: [{
                text: 'Go',
                onClick : function () {
                    // here i want to access the value of the form field 
                    // how can i access the fromdate value so that i pass it to grid 
                    console.log(this.getForm());
                    var win = new Ext.Window({
                         items:{xtype:'DashBoardGrid',fromdate:this}
                    });
                    win.show();
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            }]
        }; // eo config object

        // apply config
        Ext.apply(this, Ext.apply(this.initialConfig, config));

        Application.DashBoardForm.superclass.initComponent.apply(this, arguments);
    } // eo function initComponent
   ,onRender:function() {
        // this.store.load();
        Application.DashBoardForm.superclass.onRender.apply(this, arguments);
    } // eo function onRender
});

Ext.reg('DashBoardForm', Application.DashBoardForm);

How can I pass the value of from date here in onclick function?

Being that you gave the field an ID of 'fromdate', you can reference it using Ext.getCmp() and from there call its getValue() method:

var field = Ext.getCmp('fromdate');

var win = new Ext.Window({
    items: {
        xtype: 'DashBoardGrid',
        fromdate: field.getValue()
    }
});

Set the scope of your button 'Go', so that you will have access to form within the handler method. By doing this, you will have access to the form from the handler method.

Now, to get access to the form element, you can use ref property or use find*() methods available in Ext.form.FormPanel to get the form element.

text: 'Go',
scope: this,
handler: function () {

    fromdate = this.findById('fromdate');

    // extract date value and use it...
    value = fromdate.getValue();

}

When using ref property, set a ref for the formdata field:

ref: '../formdate'
fieldLabel: 'Start Date',
name: 'fromdate',
id: 'fromdate',
vtype: 'daterange',
value : new Date(),
endDateField: 'todate' // id of the end date field

And you should be able to access the form element through the form object in the handler.

this.formdate.getValue()