The field component in Ext JS provides some client-side validation capability. This
can save time and also bandwidth (the system will only make a server request when
it is sure the information has passed the basic validation). It also helps to point out
to the user where they have gone wrong in filling out the form. Of course, it is also
good to validate the information again on the server side for security reasons, but for
now we will focus on the validations we can apply to the form of our Login window.