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

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.