What is the difference between def and defonce in Clojure?

Question

What is the difference between def and defonce in Clojure?

When to use def over defonce or vice versa?

Answer 1

defonce is skipped when variable is already defined.

user> (def a 1) ;;=> #'user/a user> a ;;=> 1 user> (def a 2) ;;=> #'user/a user> a ;;=> 2 user> (defonce b 1) ;;=> #'user/b user> b ;;=> 1 user> (defonce b 2) ;;=> nil user> b ;;=> 1