[ruby-on-rails] How to understand nil vs. empty vs. blank in Ruby

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?

Just extend Julian's table:

Ref: empty?blank?nil???????

Quick tip: !obj.blank? == obj.present?

Can be handy/easier on the eyes in some expressions

Everybody else has explained well what is the difference.

I would like to add in Ruby On Rails, it is better to use obj.blank? or obj.present? instead of obj.nil? or obj.empty?.

obj.blank? handles all types nil, '', [], {}, and returns true if values are not available and returns false if values are available on any type of object.

exists? method can be used to check whether the data exists in the database or not. It returns boolean values either true or false.

• Everything that is nil? is blank?
• Everything that is empty? is blank?
• Nothing that is empty? is nil?
• Nothing that is nil? is empty?

tl;dr -- only use blank? & present? unless you want to distinguish between "" and " "

A special case is when trying to assess if a boolean value is nil:

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

In this case the recommendation would be to use .nil?

Just a little note about the any? recommendation: He's right that it's generally equivalent to !empty?. However, any? will return true to a string of just whitespace (ala " ").

And of course, see the 1.9 comment above, too.

I made this useful table with all the cases:

blank?, present? are provided by Rails.

One difference is that .nil? and .empty? are methods that are provided by the programming language Ruby, whereas .blank? is something added by the web development framework Rails.

Rails 4

an alternative to @corban-brook 's 'Array gotcha: blank?' for checking if an arrays only holds empty values and can be regarded as blank? true:

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

one could also do:

[nil, '', "", " ",' '].reject(&:blank?).blank? == true

nil? is a standard Ruby method that can be called on all objects and returns true if the object is nil:

b = nil
b.nil? # => true

empty? is a standard Ruby method that can be called on some objects such as Strings, Arrays and Hashes and returns true if these objects contain no element:

a = []
a.empty? # => true

b = ["2","4"]
b.empty? # => false

empty? cannot be called on nil objects.

blank? is a Rails method that can be called on nil objects as well as empty objects.

Don't forget any? which is generally !empty?. In Rails I typically check for the presence of something at the end of a statement with if something or unless something then use blank? where needed since it seems to work everywhere.

nil? can be used on any object. It determines if the object has any value or not, including 'blank' values.

For example:

example = nil
example.nil?  # true

"".nil?  # false

Basically nil? will only ever return true if the object is in fact equal to 'nil'.

empty? is only called on objects that are considered a collection. This includes things like strings (a collection of characters), hashes (a collection of key/value pairs) and arrays (a collection of arbitrary objects). empty? returns true is there are no items in the collection.

For example:

"".empty? # true
"hi".empty?   # false
{}.empty?  # true
{"" => ""}.empty?   # false
[].empty?   # true
[nil].empty?  # false

nil.empty?  # NoMethodError: undefined method `empty?' for nil:NilClass

Notice that empty? can't be called on nil objects as nil objects are not a collection and it will raise an exception.

Also notice that even if the items in a collection are blank, it does not mean a collection is empty.

blank? is basically a combination of nil? and empty? It's useful for checking objects that you assume are collections, but could also be nil.