What does ||= (or-equals) mean in Ruby?

Question

What does the following code mean in Ruby?

||=

Does it have any meaning or reason for the syntax?

Answer 1

It's like lazy instantiation. If the variable is already defined it will take that value instead of creating the value again.

Answer 2

This ruby-lang syntax. The correct answer is to check the ruby-lang documentation. All other explanations obfuscate.

Google

"ruby-lang docs Abbreviated Assignment".

Ruby-lang docs

https://docs.ruby-lang.org/en/2.4.0/syntax/assignment_rdoc.html#label-Abbreviated+Assignment

Answer 3

Basically,

x ||= y means

if x has any value leave it alone and do not change the value, otherwise set x to y

Answer 4

This is the default assignment notation

for example: x ||= 1
this will check to see if x is nil or not. If x is indeed nil it will then assign it that new value (1 in our example)

more explicit:
if x == nil
x = 1
end

Answer 5

In short, a||=b means: If a is undefined, nil or false, assign b to a. Otherwise, keep a intact.

Answer 6

Suppose a = 2 and b = 3

THEN, a ||= b will be resulted to a's value i.e. 2.

As when a evaluates to some value not resulted to false or nil.. That's why it ll not evaluate b's value.

Now Suppose a = nil and b = 3.

Then a ||= b will be resulted to 3 i.e. b's value.

As it first try to evaluates a's value which resulted to nil.. so it evaluated b's value.

The best example used in ror app is :

#To get currently logged in iser
def current_user
  @current_user ||= User.find_by_id(session[:user_id])
end

# Make current_user available in templates as a helper
helper_method :current_user

Where, User.find_by_id(session[:user_id]) is fired if and only if @current_user is not initialized before.