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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?

Solution

  • This question has been discussed so often on the Ruby mailing-lists and Ruby blogs that there are now even threads on the Ruby mailing-list whose only purpose is to collect links to all the other threads on the Ruby mailing-list that discuss this issue.

    Here's one: The definitive list of ||= (OR Equal) threads and pages

    If you really want to know what is going on, take a look at Section 11.4.2.3 "Abbreviated assignments" of the Ruby Language Draft Specification.

    As a first approximation,

    a ||= b

    is equivalent to

    a || a = b

    and not equivalent to

    a = a || b

    However, that is only a first approximation, especially if a is undefined. The semantics also differ depending on whether it is a simple variable assignment, a method assignment or an indexing assignment:

    a    ||= b
a.c  ||= b
a[c] ||= b

    are all treated differently.