nil? empty? blank?

nil-empty-blank.md

.nil?

• It is Ruby method
• It can be used on any object and is true if the object is nil.
• "Only the object nil responds true to nil?" - RailsAPI

nil.nil? = true
anthing_else.nil? = false
a = nil
a.nil? = true
“”.nil = false

.empty?

• It is Ruby method
• 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

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

.blank?

• It is Rails method
• 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

• It also evaluates true on strings which are non-empty but contain only whitespace:

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

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

activesupport/lib/active_support/core_ext/object/blank.rb, line 17 # (Ruby 1.9)

def present? 
 !blank?
end

source http://pramodtech.quora.com/nil-empty-blank-present-in-Ruby-on-Rails