A concise explanation of nil v. empty v. blank in

Question

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 1

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

Can be handy/easier on the eyes in some expressions

Answer 2

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.

Answer 3

• 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 " "

Answer 4

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.

Answer 5

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.

Answer 6

.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 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

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