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

Solution 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 

Solution 2:

I made this useful table with all the cases:

blank?, present? are provided by Rails.

Solution 3:

Just extend Julian's table:

Ref: empty?blank?nil?傻傻分不清楚

Solution 4:

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

Can be handy/easier on the eyes in some expressions

Solution 5:

• 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 " "