Ruby and Rails have several methods that can be used to check for the existence of a value or the state of an object. Ruby provides #nil? and #empty?, and Rails' ActiveSupport adds #blank? and #present?. All these work in their own way and it's important to know how each evaluates data as using the wrong method in your code might cause unexpected results.

In this article, we'll refresh your knowledge on these methods. We'll (re)learn what conditions pass or fail when each is used as well as the type of objects each method can be used on. We'll even throw in a handy cheat sheet at the end!

#nil?

#nil? is a Ruby method on the Object class. Since all classes inherit from this class, #nil? can be used on any object. It returns true for nil (an instance of the class NilClass) and false for everything else.

nil.nil?
=> true
 
true.nil?
=> false
 
5.nil?
=> false
 
"".nil?
=> false
 
[].nil?
=> false

#empty?

#empty? is a method that can be used on strings, arrays, hashes and sets. It returns true if the instance of the object has a length of zero.

When used on strings, it returns true for empty strings.

"".empty?
=> true

Note that whitespace characters are characters and so using #empty? on a string containing only whitespace will return false. Because of this, it might not be the best method to use when validating text. If you are only using this method to validate user input, for instance, you might end up processing input that only contains whitespace.

" ".empty?
=> false
 
"\t\n".empty?
=> false

For arrays, hashes and sets, it returns true if they have no elements.

[].empty?
=> true
 
{}.empty?
=> true
 
require 'set'
Set.new.empty?
=> true

When #empty? is used on nil or any other class that doesn't have the method, a NoMethodError exception will be raised. So to use it in checking your data, you have to put in place more checks to ensure your program never crashes.

if !my_var.nil? && !my_var.empty?
  # make use of my_var
end

A better method to use would be the Rails #blank? method, which we'll now look at.

#blank?

#blank? is a Rails method (in ActiveSupport). It operates on any object.

For strings, it will return true for empty strings as well as strings containing only whitespace characters.

"".blank?
=> true
 
" ".blank?
=> true
 
"\n\t".blank?
=> true

For arrays, hashes and sets, it works just like #empty?, in that it returns true if they have no elements.

[].blank?
=> true
 
{}.blank?
=> true
 
Set.new.blank?
=> true
 
[nil].blank?
=> false
 
["", ""].blank?
=> false
 
person = {:firstName => "John", :lastName => "Doe"}
person.blank?
=> false

It returns false for true and true for any falsey conditions (i.e. nil and false).

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

As you can see, it is better to use #blank? rather than #empty? in validating data as it won't cause NoMethodErrors when used on some objects since it is available for all objects. It also won't pass for strings that only contain whitespace.

#present?

#present? is also a Rails method. It does the opposite of what #blank? does.

!my_var.blank? == my_var.present?
=> true

Here are some examples:

"".present?
=> false
 
" ".present?
=> false
 
[].present?
=> false
 
nil.present?
=> false
 
true.present?
=> true
 
false.present?
=> false
 
{}.present?
=> false
 
person = {:firstName => "John", :lastName => "Doe"}
person.present?
=> true
 
5.present?
=> true

This is also a better method for validation than #empty? for the same reasons we covered for #blank?.

In Conclusion

Since the naming of these methods tends to confuse, here's a handy cheat sheet for you to bookmark.

As with everything, the choice between #nil?, #empty?, #blank? and #present? depends on the situation. For checking if a user has a name to address them with, #present? could be a good fit, for example. However #blank? and #present return a boolean for all objects, which might hide bugs in the code by not raising an error when a value is of the wrong type.

We hope we refreshed your memory on checking values in Ruby. If you have any questions or comments, don’t hesitate to drop us a line @AppSignal.