The OnValueChanging client-side event handler is called when the user changes the value of the input control. The event occurs immediately after the control loses focus, but before its value has been updated.
Two parameters are passed to the event handler:
sender is the input control.
eventArgs has the following methods:
set_cancel() lets you permit or block the change to the value of the control. Calling set_cancel(true) prevents the change.
get_oldValue() returns the string value of the input control before the user made any changes.
get_newValue() returns the string value that is about to be assigned to the input control.
set_newValue() lets you change the value that is about to be assigned to the input control.