Conditional Assignment in Ruby

The a ||= b is a conditional assignment operator. It’s equivalent to a || a = b.

If a is false, i.e.: nil or false, then evaluate b and assign the result to a.

If a is truthy, that is, other than nil and false, b will not be evaluated, and a will remain unchanged.

For example:

a = nil
a ||= 10
a
=> 10

a = 1
a ||= 10
a
=> 1

The gotcha is: if a += b equals a = a + b then a ||= b should be a || a = b.

It is an entirely incorrect assumption. If a is not declared beforehand, a || a = b will result in an undefined variable error.

a || a = 10

Output:

NameError: undefined local variable or method `a'