Working with listeners

Showing an alert when a textField is left blank after clicking ok of dialog box, Listener attached to the dialog :

Code in listeners (Javascript) :

 function(dialog) {

var textfieldArray = dialog.findByType("textfield");

var data = textfieldArray[0].getValue();

if(data == "")

{

var  property= this.find("name","./title")[0];

property.allowBlank=false;

alert("The field is empty");

alertDialog.show();

}

}