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?

One gotcha is an empty array returns false for present?. Which is because blank? returns true for an empty array. — Kris
– Kris, Commented Apr 25, 2012 at 10:41
One nice thing is to see that :nil? is defined on ::Kernel and overridden on ::NilClass, while :empty? is implemented separately on many classes (natively on ::String, ::Array, ::Hash, and non-natively on other classes like ::Set from stdlib and ::ActiveRecord::Relation from rails). So :nil? is available in all subclasses of ::Object and also in every class that includes ::Kernel by itself, where :empty? must be implemented or included specifically in your classes. — rewritten
– rewritten, Commented Mar 22, 2016 at 10:00
If you try to understand the whole nil concept start here. — totymedli
– totymedli, Commented Jun 13, 2018 at 18:07

alhimik45 · Accepted Answer · 2015-10-06 11:39:24Z

1501

.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

edited Oct 6, 2015 at 11:39

alhimik45

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answered May 20, 2009 at 16:14

Corban Brook

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10 Comments

Andrew Grimm Over a year ago

As mentioned in the question, some non-empty strings count as blank.

tillda Over a year ago

Why haven't they overwritter nil's .empty? if they could add .blank?

Shannon Over a year ago

Also: false.blank? == true

TrinitronX Over a year ago

I've seen blank? erroneously used in non-rails contexts before. If you really need it you can include it: require 'active_support/core_ext/object/blank'

Houen Over a year ago

As an addition to the all-blank-array gotcha that Tom is mentioning in his answer, be aware that this can lead to security issues where an attacker can use unprepared code that simply checks for unless params.nil? or unless params.blank?, and is the cause of much discussion and work currently in Rails. See here for more information: github.com/rails/rails/issues/13420

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dimpiax · Accepted Answer · 2018-08-12 16:19:17Z

716

I made this useful table with all the cases:

enter image description here

blank?, present? are provided by Rails.

edited Aug 12, 2018 at 16:19

dimpiax

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answered Dec 18, 2013 at 16:29

Julian Popov

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3 Comments

Lonny Eachus Over a year ago

It should be noted that blank? and present? are Rails-only. They return Exception: NoMethodError in Ruby 2.0.0.

Dennis Over a year ago

Consider adding any? if you ever update the table.

the Tin Man Over a year ago

blank? and present? are not "Rails-only", they're automatically included by Rails but can easily be included in non-Rails code by using ActiveSupport's Core extensions. See stackoverflow.com/questions/4238867/…

Sibevin Wang · Accepted Answer · 2016-09-04 14:09:37Z

231

Just extend Julian's table:

Ref: empty?blank?nil?傻傻分不清楚

edited Sep 4, 2016 at 14:09

answered Nov 12, 2014 at 0:20

Sibevin Wang

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4 Comments

Lahiru Jayaratne Over a year ago

@Sibevin Wang, I have downloaded this table and used it in some of my answers adding the credits to you. Hope you won't mind :) Kudos to this great effort!

daniero Over a year ago

Enumerable#any? is vanilla Ruby, it shouldn't be listed under "rails only"

Sibevin Wang Over a year ago

@daniero Thanks for your reminder, the table is updated.

DaveMongoose Over a year ago

Numeric types also have .zero? and .nonzero?

Alexander Malfait · Accepted Answer · 2010-07-13 11:33:23Z

66

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

Can be handy/easier on the eyes in some expressions

answered Jul 13, 2010 at 11:33

Alexander Malfait

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6 Comments

Kirk Over a year ago

is there something equivalent to !obj.nil?

Factor Mystic Over a year ago

present? is part of active_support, check here on what to include to get it: stackoverflow.com/a/4648704/1569

Mohamed Hafez Over a year ago

@Kirk: !!obj is equivalent to !obj.nil?

Ajedi32 Over a year ago

@MohamedHafez No it's not. !!false

Mohamed Hafez Over a year ago

Agh, @Ajedi32 good catch! equivalent with that one exception.

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steve · Accepted Answer · 2018-01-15 18:17:51Z

59

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 " "

answered Jan 15, 2018 at 18:17

steve

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1 Comment

Jason May 15 at 20:38

Or if you want to use logic that executes on either nil? or empty? and don't want to deal with whether the contents of the variable respond to empty?.

the Tin Man · Accepted Answer · 2012-11-14 14:53:41Z

57

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.

edited Nov 14, 2012 at 14:53

the Tin Man

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answered May 19, 2009 at 23:42

Andrew Grimm

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Comments

Alan H · Accepted Answer · 2012-11-14 13:56:02Z

19

A special case is when trying to assess if a boolean value is nil:

false.present? == false
false.blank? == true
false.nil? == false

In this case the recommendation would be to use .nil?

answered Nov 14, 2012 at 13:56

Alan H

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Comments

davemyron · Accepted Answer · 2009-07-13 19:25:16Z

16

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.

answered Jul 13, 2009 at 19:25

davemyron

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1 Comment

Lonny Eachus Over a year ago

In Rails, any string containing just whitespace characters is "blank": " \n\r\t".blank? #=> true

AndyAndy · Accepted Answer · 2009-05-19 23:51:50Z

15

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.

answered May 19, 2009 at 23:51

Andy

2 Comments

Andrew Grimm Over a year ago

.any? doesn't work with strings in ruby 1.9, as .any? requires enumerable, and string#each by itself doesn't work in ruby 1.9.

Kelvin Over a year ago

Careful about any? for checking non-emptiness. It returns false if you have an array of falsy values. A way to get around that is to pass a always-true block: [nil, false].any?{ true }, but !empty? is shorter and you don't have remember the weird behavior.

the Tin Man · Accepted Answer · 2016-01-29 18:27:21Z

14

nil? is a standard Ruby method that can be called on all objects and returns true if the object is nil:

b = nil
b.nil? # => true

empty? is a standard Ruby method that can be called on some objects such as Strings, Arrays and Hashes and returns true if these objects contain no element:

a = []
a.empty? # => true

b = ["2","4"]
b.empty? # => false

empty? cannot be called on nil objects.

blank? is a Rails method that can be called on nil objects as well as empty objects.

edited Jan 29, 2016 at 18:27

the Tin Man

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answered Dec 22, 2014 at 5:41

neha

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Comments

the Tin Man · Accepted Answer · 2016-01-29 18:25:33Z

9

Everybody else has explained well what is the difference.

I would like to add in Ruby On Rails, it is better to use obj.blank? or obj.present? instead of obj.nil? or obj.empty?.

obj.blank? handles all types nil, '', [], {}, and returns true if values are not available and returns false if values are available on any type of object.

edited Jan 29, 2016 at 18:25

the Tin Man

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answered May 20, 2015 at 7:55

Manish Shrivastava

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Comments

stealththeninja · Accepted Answer · 2023-05-16 16:07:51Z

Though there n-number of answers available to this question but I liked the way its being explained here so posted one more answer :-)

Look at the data metric below - Its self explanatory with the various data-types used across available methods for it.

	#nil?	#empty?	#blank?	#present?
5	false	NoMethodError	false	true
""	false	true	true	false
" "	false	false	true	false
"\t\n"	false	false	true	false
[]	false	true	true	false
["a"]	false	false	false	true
{}	false	true	true	false
{a: "b"}	false	false	false	true
Set.new	false	true	true	false
nil	true	NoMethodError	true	false
true	false	NoMethodError	false	true
false	false	NoMethodError	true	false

Reference: https://blog.appsignal.com/2018/09/11/differences-between-nil-empty-blank-and-present.html

ndnenkov · Accepted Answer · 2016-02-18 19:05:37Z

7

exists? method can be used to check whether the data exists in the database or not. It returns boolean values either true or false.

edited Feb 18, 2016 at 19:05

ndnenkov

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answered Feb 17, 2016 at 11:25

kelvin

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1 Comment

Roger Over a year ago

Works only on model class!

mahatmanich · Accepted Answer · 2020-05-25 09:36:48Z

6

Rails 4

an alternative to @corban-brook 's 'Array gotcha: blank?' for checking if an arrays only holds empty values and can be regarded as blank? true:

[ nil, '' ].all? &:blank? == true

one could also do:

[nil, '', "", " ",' '].reject(&:blank?).blank? == true

edited May 25, 2020 at 9:36

answered Aug 19, 2015 at 13:52

mahatmanich

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Comments

the Tin Man · Accepted Answer · 2016-01-29 18:23:46Z

nil? can be used on any object. It determines if the object has any value or not, including 'blank' values.

For example:

example = nil
example.nil?  # true

"".nil?  # false

Basically nil? will only ever return true if the object is in fact equal to 'nil'.

empty? is only called on objects that are considered a collection. This includes things like strings (a collection of characters), hashes (a collection of key/value pairs) and arrays (a collection of arbitrary objects). empty? returns true is there are no items in the collection.

For example:

"".empty? # true
"hi".empty?   # false
{}.empty?  # true
{"" => ""}.empty?   # false
[].empty?   # true
[nil].empty?  # false

nil.empty?  # NoMethodError: undefined method `empty?' for nil:NilClass

Notice that empty? can't be called on nil objects as nil objects are not a collection and it will raise an exception.

Also notice that even if the items in a collection are blank, it does not mean a collection is empty.

blank? is basically a combination of nil? and empty? It's useful for checking objects that you assume are collections, but could also be nil.

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