How to understand nil vs. empty vs. blank in Ruby

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

.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  

Answer 2

I made this useful table with all the cases:

blank?, present? are provided by Rails.