Ruby Quiz

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What's the return value of the following Ruby code?

y = false
y ||= 42

y # => ???

The correct answer is

nil

false

42

true

Let’s analyze this simple snippet:

y = false
y ||= 42

# => 42

||= is shorthand for:

y = y || 42

So Ruby evaluates y || 42. In Ruby, only two values are falsy: false and nil.

Everything else (0, "", [], {}, etc.) is truthy.

On the first line, y is assigned false.

Then y ||= 42 is the same as y = (false || 42).

false || 42 evaluates to 42.

So y now holds 42.

Here is quick note on Logical OR algebra

The logical OR operator || follows this algebra:

true || anything → true

false || x → x

This is why false || 42 gives 42.

Many expect false to “stick,” but with ||= Ruby treats false just like nil: it triggers assignment of the fallback value.

x = nil
x ||= 100

x # => 100

a = "hello"
a ||= "world"

a # => "hello"

If the variable is nil or false, the fallback is assigned.

If the variable is any truthy value, it stays as it is.