Interactive rating component

I couldn't figure out how to prevent user from submitting without having rating selected, but I didn't know how to check if rating button is not clicked and display an alert message. Tried also to check if :focus on any button is active and if not, then display an alert, but I couldn't reach it in JS. My next idea was to change buttons into radio inputs, hide them and catch their labels as buttons, but I think a label can't have an active state like :focus so I gave up. Any ideas how to solve the problem? Also any other feedback and advice would be highly appreciated.