What does ||= do in Ruby

Question

I've been using Ruby for a while now, and I keep seeing this:

foo ||= bar

What is it?

score 9 · Accepted Answer · answered Jan 08 '10 at 03:01

9

This will assign bar to foo if (and only if) foo is nil or false.

EDIT: or false, thanks @mopoke.

answered Jan 08 '10 at 03:01

Peter

score 7 · Answer 2 · answered Jan 08 '10 at 03:01

7

Operator ||= is a shorthand form of the expression:

x = x || "default"

Operator ||= can be shorthand for code like:

x = "(some fallback value)" if x.nil?

answered Jan 08 '10 at 03:01

shedd

score 2 · Answer 3 · answered Jan 08 '10 at 03:01

2

Assign bar to foo unless foo is a true value (not false or nil).

answered Jan 08 '10 at 03:01

mopoke

2

Should be: unless `foo` is truthy, where truthy means: not `false` or `nil`. – yfeldblum Jan 08 '10 at 03:04

score 1 · Answer 4 · answered Jan 10 '10 at 22:22

If you're using it for an instance variable, you may want to avoid it. That's because

@foo ||= bar

Can raise a warning if @foo was previously uninitialized. You may want to use

@foo = bar unless defined?(@foo)

or

@foo = bar unless (defined?(@foo) and @foo)

depending on whether you want to merely check if @foo is initialized, or check if @foo has truthiness (ie isn't nil or false).

4 Answers4