The hardest thing for me to grok in Ruby

the_hardest_thing_for_me_to_grok_in_ruby.rb

colors = ["red"]   #=> ["red"]
colors_2 = colors  #=> ["red"]
colors_2 << "blue" #=> ["red", "blue"]
colors             #=> ["red", "blue"]

def add_white (original_colors)
  original_colors << "white"
end

add_white colors   #=> ["red", "blue", "white"]
colors_2           #=> ["red", "blue", "white"]

TL;DR: hashes have some sharp edges because depending on assignment it is possible for them to mutate out from under you as you pass them around. It is also possible for them not to mutate out from under you.

• https://rosettacode.org/wiki/Address_of_a_variable#Ruby
• https://launchschool.com/blog/mutating-and-non-mutating-methods
• https://launchschool.com/blog/references-and-mutability-in-ruby
• http://rubyblog.pro/2017/09/pass-by-value-or-pass-by-reference
• http://nicholasjohnson.com/ruby/ruby-course/exercises/hashes-and-symbols/