Fish equivalent of bash $(command) notation

I am currently trying out the fish shell instead of using bash. One type of notation I'm having trouble learning the fish-equivalent notation for is $(command), similar to how it is described in this SOF post. How do I write this using fish? Keep in mind that I could use backslash characters around the command I want to evaluate, but the linked post and other posts discourage this because it is an old style of evaluating commands.

Specifically, this is the bash command I want to convert to fish syntax (for initializing rbenv during startup of the shell):

eval "$(rbenv init -)"