Scenario is there is case object there is no any visual force page or trigger. i need to show alert box when user is edited the record of employeeId this is text field show alert message that "this employeeId doesn't exceed do you want to continue"
There's an idea or two for this (actually, more like 16 or so), and is currently under Product Management review. In the meantime, you can either use possibly a validation rule or trigger (which will block the save entirely until corrected), or you could write a Visualforce page or Lightning Component and override the default new/edit buttons. I'd recommend voting on the ideas so that we can have this as a standard feature without coding.