A person who is on hand and ready to perform a service or respond to an emergency; An unusually alert fellow who is capable of decisive action, seizing an opportunity.
When you need help, call me and I will be your Johnny-on-the-spot" "Oooh, thank you, girl!
Something, or some person performing an action in precise manner as expected by another party.
Being on time or in the correct place.
Hector, You were hella johnny-on-the-spot with those fries when I got the munchies.