A concise explanation of nil v. empty v. blank in Ruby on Rails

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I find myself repeatedly looking for a
clear definition of the differences of nil?,
blank?, and empty? in Ruby on Rails.
Here's the closest I've
come:

• blank?
  objects are false, empty, or a whitespace string. For example,
  "", " ", nil,
  [], and {} are
  blank.




• nil?
  objects are instances of
  NilClass.




• empty?
  objects are class-specific, and the definition varies from class to class. A string is
  empty if it has no characters, and an array is empty if it contains no
  items.

Is
there anything missing, or a tighter comparison that can be made?

Answer

.nil? can be used on
any object and is true if the object is
nil.

.empty? can be
used on strings, arrays and hashes and returns true
if:

• String length ==
  0



• Array length ==
  0


• Hash length ==
  0

Running
.empty? on something that is nil will throw a
NoMethodError.

That is
where .blank? comes in. It is href="http://apidock.com/rails/Object/blank%3F" rel="noreferrer">implemented by
Rails and will operate on any object as well as work like
.empty? on strings, arrays and
hashes.

nil.blank? ==
            true
false.blank? == true

[].blank? ==
            true
{}.blank? == true
"".blank? == true
5.blank? ==
            false
0.blank? ==
            false

.blank?
also evaluates true on strings which are non-empty but contain only
whitespace:

" ".blank? ==
            true

" ".empty? ==
            false

href="http://apidock.com/rails/Object/presence" rel="noreferrer">Rails also
provides .present?, which returns the negation of
.blank?.

Array gotcha:
blank? will return false even if all
elements of an array are blank. To determine blankness in this
case, use all? with blank?, for
example:

[ nil, '' ].blank? ==
            false
[ nil, '' ].all? &:blank? == true