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?

asked May 19 '09 at 22:51

Arrel
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Corban Brook · Answer 1 · 2009-05-21 19:48:47Z

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

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

"  ".blank? == true
"  ".empty? == false

Andrew Grimm · Answer 2 · 2009-05-19 23:42:49Z

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.

Andy · Answer 3 · 2009-05-19 23:51:50Z

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.

orangechicken · Answer 4 · 2009-07-13 19:25:16Z

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.

score 0 · Answer 5

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

Can be handy/easier on the eyes in some expressions

rails nil blank

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

5 Answers