nil value, nil object
The nil value is used as a concept to express the meaning of “the absence of value” or “no object” or “a missing object” or “a lack of an object”.
In other programming languages it is often called null.
Like any other value in Ruby, nil is an object. It is an object of the class NilClass.
myarray =  myarray # => nil nil.class # => NilClass
The nil? method is defined for any object.
All objects except nil return false.
You can use this method to test if an object is nil.
false.nil? # => false 0.nil? # => false nil.nil? # => true myarray =  myarray == nil # => true myarray.nil? # => true
Inside a boolean expression nil is treated as false.
In Ruby false and nil are the only values (objectes) to be considered “falsy”. Any other value (object) is considered to be “truthy”.
!! nil # => false not not nil # => false myarray =  myarray ? 'Element at index 0 exists' : 'There is no element at index 0' # => "There is no element at index 0"