What is the difference between nil, empty, and blank in Ruby?

nil means no value at all. empty? asks whether a string or collection has no elements, and blank? is a Rails check that also treats nil and whitespace as empty.

• nil is the single object of NilClass. Only nil returns true for nil?, and nil has no empty? method.
• empty? is defined on String, Array and Hash. It is true only when the length is zero, so " ".empty? is false.
• blank? comes from Rails' Active Support, not plain Ruby. It is true for nil, false and empty collections. A string of only whitespace is blank too.

Plain Ruby treats only nil and false as falsy. An empty string and 0 are truthy, which is why blank? is useful for form input.

nil.nil?        # true
"".empty?       # true
[].empty?       # true
" ".blank?      # true (Rails only)