REQUIRED

Hi!

I want to make this Scenario:

When User input in the Name Field I will add a javascript that will validate if the user has filled up the Name Field and automatically make the Last Name Required. If the User Clear the Name Field automatically the Last Name will not be Required.

Just want to know what is the trigger of required? Is it a class or something that I can use to add in a field to make it required Using Javascript.

Thanks!