Ruby is pass reference by value

value_or_reference.rb

def value_and_identity(object)
  "(Value: #{object.inspect}, ID: #{object.object_id})"
end

def increment(val)
  puts "---> #increment received argument #{value_and_identity(val)}"
  result = val += 1
  puts "---> 'result' is #{value_and_identity(result)}"
  result
end

def change_string(str)
  puts "---> #change_string received argument 'str': #{value_and_identity(str)}"
  str << "Mutating this string! "
  str << "And the outside world will see it..."
  puts "---> 'str' is #{value_and_identity(str)}"
  str = "...because objects in Ruby are Reference types" # Their value is a reference to another object
end

if __FILE__ == $0
  puts "Ruby is 'pass reference by value'..."

  puts "Example 1:"
  array_a = [0]
  puts [value_and_identity(array_a)]
  array_b = array_a
  puts [value_and_identity(array_a), value_and_identity(array_b)]
  array_b << 1
  puts [value_and_identity(array_a), value_and_identity(array_b)]
  puts "\n\n"

  puts "Example 2:"
  val = 1
  puts "-> Passing variable 'val' #{value_and_identity(val)} to #increment function..."
  increment(val)
  puts "-> after calling #increment with 'val': #{value_and_identity(val)}"
  puts "\n\n"

  puts "... Except all object values are references..."
  puts "\n\n"

  puts "Example 3:"
  str = "hello"
  puts "-> Passing variable 'str' #{value_and_identity(str)} to #change_string function..."
  change_string(str)
  puts "-> after calling #change_string with 'str': #{value_and_identity(str)}"
  puts "\n\n"

  puts "If you want simpler Ruby programs, prefer immutability."
  
end

$ ruby value_or_reference.rb
Ruby is 'pass reference by value'...
Example 1:
(Value: [0], ID: 2156900320)
(Value: [0], ID: 2156900320)
(Value: [0], ID: 2156900320)
(Value: [0, 1], ID: 2156900320)
(Value: [0, 1], ID: 2156900320)


Example 2:
-> Passing variable 'val' (Value: 1, ID: 3) to #increment function...
---> #increment received argument (Value: 1, ID: 3)
---> 'result' is (Value: 2, ID: 5)
-> after calling #increment with 'val': (Value: 1, ID: 3)


... Except all object values are references...


Example 3:
-> Passing variable 'str' (Value: "hello", ID: 2156899040) to #change_string function...
---> #change_string received argument 'str': (Value: "hello", ID: 2156899040)
---> 'str' is (Value: "helloMutating this string! And the outside world will see it...", ID: 2156899040)
-> after calling #change_string with 'str': (Value: "helloMutating this string! And the outside world will see it...", ID: 2156899040)


If you want simpler Ruby programs, prefer immutability.