what does a ||= mean in Ruby language? [duplicate]

Question

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What does ||= mean in Ruby?

what does the below line mean?

a ||= {} 
a ||= 1

in irb it always returns the class of a, as hash, for both the above lines. Thanks in advance.

Answer 1

||= is an assignment operator, which returns the value assigned. a ||= b is equivalent to the statement a || a = b which means that if a is set and has some true value, then it remains the same, otherwise it takes the value of b.

In your example a is only ever set once, which explains the behaviour you've noticed.

a ||= {} 
a ||= 1 // a is still {}

Typical usage I've seen is to initialise static variables, ie.

class Foo
    def self.bar
        return @bar ||= {}
    end
end

EDIT:

It bears mentioning that ||= is a short-circuit operator. This means that it in the case of a ||= b there will only be an assignment of a = b. There will never be an assignment of a = a in the case that a is non-false. This is a little pedantic, but matters in some (very) edge cases.

For more information, read the definitive list of ||= threads and pages.