Creating alerts for required fields

I have created a booking form with several required fields.
But when the user tries to submit an uncompleted form, the alerts received are less than meaningfull.

Example: I create a required field called ‘Name’
If the user tries to submit the form without filling that field, a js alert box emerges with the text: “Name* : undefined”

What can I do – short of changing the core code – to create meaningfull alerts like “You must fill the field Name” ?

Best regards