NoMethodError: undefined method for nil:NilClass... Explained

#rails #ruby #errorexplained #beginners

This is a common Ruby error which indicates that the method or attribute for an object you are trying to call on an object has not been defined.

NoMethodError: undefined method SOME_METHOD for nil:NilClass

For example, the String class in Ruby has the method size (which is synonymous with length, so I can write...

greeting = "hello"
greeting.size
#=> 5

But loveliness does not exist for a string, so when I type it, I will get...

NoMethodError: undefined method loveliness for nil:NilClass

I find this will come up when I think I'm operating on an object with methods, but I'm actually operating on a hash with attributes.

my_hash[:loveliness]
# "very lovely" yay, this is a thing that exists!
my_hash.loveliness
# NoMethodError: undefined method loveliness nil:NilClass

Of course, because this is Ruby, we can actually define loveliness if we wanted to very easily by monkeypatching the String class.

class String

  def loveliness
    "very lovely"
  end

end

Now we know how lovely our string is. If I was mistaken or unclear, please feel free to add a comment to clarify anything here.

Happy coding ❤️

Top comments (5)

Marc Köhlbrugge • Jul 2 '20 • Edited

This one tripped up me all the time when I was just starting out. While the error seems to be about the method, it's usually the object you're trying to call it on that is problematic somehow.

If the object is allowed to be nil, you want to check its presence before calling any method on it. For example:

coffee.drink_it if coffee.present?

Or if you like concise code:

coffee&.drink_it 
# shorthand for `coffee && coffee.drink_it`
# which basically does the same thing as the present? check

Small nitpick about the example code in article: if my_hash was an actual hash, the error would say {:foo => :bar}:Hash instead of nil:NilClass.