What does ||= (or-equals) mean in Ruby?
What does the following code mean in Ruby?
||=
Does it have any meaning or reason for the syntax?
Answer
a ||= b is a conditional assignment operator. It means if a is undefined or falsey, then evaluate b and set a to the result. Equivalently, if a is defined and evaluates to truthy, then b is not evaluated, and no assignment takes place. For example:
a ||= nil # => nil
a ||= 0 # => 0
a ||= 2 # => 0
foo = false # => false
foo ||= true # => true
foo ||= false # => true
Confusingly, it looks similar to other assignment operators (such as +=), but behaves differently.
a += btranslates toa = a + ba ||= broughly translates toa || a = b
It is a near-shorthand for a || a = b. The difference is that, when a is undefined, a || a = b would raise NameError, whereas a ||= b sets a to b. This distinction is unimportant if a and b are both local variables, but is significant if either is a getter/setter method of a class.
Further reading: