The purpose of ||=

I has been asked many times, what does this (very common) syntactic trick do?

Ben Taylor provides this very succinct description:

To understand better how this construct works, try writing in irb

>> nil || "steve"
=> "steve"

>> "steve" || "john"
=> "steve"

>> "steve" || nil
=> "steve"

>> nil && "steve"
=> nil

>> "steve" && nil
=> nil

>> "steve" && "john"
=> "john"

The construct ||= is shorthand for

>> @blah = @blah || "something"
=> "something"

It's common practice in many programming languages to use boolean operators like this as shorthand for using an if statement.

>> @blah ||= "something"
=> "something"

Can also be written as

>> @blah
=> "something"
>> @blah = "something" unless @blah
=> nil
>> @blah
=> "something"

In more verbose languages it would need to be a full if statement.