Question

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What does !! mean in ruby?

I’m learning ruby/rails and found a tutorial with the following code example:

def role?(role)
    return !!self.roles.find_by_name(role.to_s.camelize)
end

I don’t have any idea for what the !! do, neither the !!self do.

I really googled about that, but doesn’t find anything.

Can anyone give a short explanation? Thanks in advance.

Answer 1

It’s the “not” operator (!) repeated twice, so that it’s argument will be coerced to its negated boolean and then its corresponding boolean. Basically, it’s a way to coerce any object into its boolean value.

!!false # => false
!!nil # => false
!!true # => true
!!{} # => true
!![] # => true
!!1 # => true
!!0 # => true (Surprised? Only 'false' and 'nil' are false in Ruby!)