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

Concise and complete answer

a ||= b

evaluates the same way as each of the following lines

a || a = b
a ? a : a = b
if a then a else a = b end

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On the other hand,

a = a || b

evaluates the same way as each of the following lines

a = a ? a : b
if a then a = a else a = b end

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Edit: As AJedi32 pointed out in the comments, this only holds true if: 1. a is a defined variable. 2. Evaluating a one time and two times does not result in a difference in program or system state.