When passing a Ruby array as an argument, why does `<<` append while `+=` does not?

Question

I ran across an unexpected result when passing an array to a function and the behavior of << vs +=.

Can anyone explain why the following

Answer 1

My understanding is that in Ruby all function arguments are passed by reference

No, as you have discovered yourself, Ruby is pass-by-value, not pass-by-reference. That's why you are seeing the result you are seeing.

Answer 2

The main difference between the #<<, and #+ s that #<< is just the Array's instance method, so you just add a value to the specified instance of Array

arr = []
arr.__id__ # => 68916130
arr << 10
arr.__id__ # => 68916130

but in form of #+ is used assignment operator, which replace reference to a variable with a new instance, and that new instance shall not be passed into uplevel of the #add_to_array function.

arr = []
arr.__id__ # => 68916130
arr += [10]
arr.__id__ # => 68725310

NOTE: That += implies the #+ method plus assignment operator =, however ruby interpreter treats it as a specific operator not as a sum.

Additionals

The form arr = arr + [10] isn't work properly also.

def add_to_array(value, arr)
  arr = arr + [value]
end
build_results()
# => []