The defined? expression allows you to test if a variable is defined or not. If it isn’t defined, it returns nil. Otherwise, it returns a string that provides information about the variable.
> defined? a
=> nil
> a = 10
=> 10
> defined? a
=> "local-variable"
> defined? nil
=> "nil"
> defined? Array
=> "constant"
> defined? String
=> "constant"
* class Person
> end
> defined? Person
=> "constant"
> defined? 10
=> "expression"
A variable set to nil is also initialized.
> b = nil
=> nil
> defined? b
=> "local-variable"