I believe the author is referencing Ruby's Enumerable class (https://ruby-doc.org/core-2.4.1/Enumerable.html https://ruby-doc.org/core-2.4.1/Enumerable.html), a

I believe the author is referencing Ruby's Enumerable class (https://ruby-doc.org/core-2.4.1/Enumerable.html https://ruby-doc.org/core-2.4.1/Enumerable.html), and getting those helpful functions in JS.