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

a ||= b is the same as saying a = b if a.nil? or a = b unless a

But do all 3 options show the same performance? With Ruby 2.5.1 this

1000000.times do
  a ||= 1
  a ||= 1
  a ||= 1
  a ||= 1
  a ||= 1
  a ||= 1
  a ||= 1
  a ||= 1
  a ||= 1
  a ||= 1
end

takes 0.099 Seconds on my PC, while

1000000.times do
  a = 1 unless a
  a = 1 unless a
  a = 1 unless a
  a = 1 unless a
  a = 1 unless a
  a = 1 unless a
  a = 1 unless a
  a = 1 unless a
  a = 1 unless a
  a = 1 unless a
end

takes 0.062 Seconds. That's almost 40% faster.

and then we also have:

1000000.times do
  a = 1 if a.nil?
  a = 1 if a.nil?
  a = 1 if a.nil?
  a = 1 if a.nil?
  a = 1 if a.nil?
  a = 1 if a.nil?
  a = 1 if a.nil?
  a = 1 if a.nil?
  a = 1 if a.nil?
  a = 1 if a.nil?
end

which takes 0.166 Seconds.

Not that this will make a significant performance impact in general, but if you do need that last bit of optimization, then consider this result. By the way: a = 1 unless a is easier to read for the novice, it is self-explanatory.

Note 1: reason for repeating the assignment line multiple times is to reduce the overhead of the loop on the time measured.

Note 2: The results are similar if I do a=nil nil before each assignment.