Truthiness: only false and nil

Only false and nil are falsy

Every Ruby value counts as true in a condition except two: false and nil. nil means no value, the answer you get when something is missing.

Both lines below stay silent because their conditions are falsy, so only the last line prints:

puts "hi" if false puts "hi" if nil puts "done"

done

Zero is truthy

In some languages zero behaves like false, but not in Ruby. The number zero is an ordinary truthy value, so a counter reaching zero still passes the check.

if 0 puts "zero is truthy" end

zero is truthy

Empty is truthy

Empty values are truthy as well. An empty string and an empty array both pass the check: only false and nil can stop a condition.

if "" puts "empty string is truthy" end if [] puts "empty array is truthy" end

empty string is truthy
empty array is truthy

Use it for guards

Truthiness shines in small guard lines that handle the missing case first. Ask whether a value is nil, print a fallback, and let the rest of the code assume a real value.

name = nil puts "guest" if name.nil?

guest

Flip the variable to a real string and the guard stays silent. Lesson 10 builds full if and else branches on top of this idea.

Next: nil and object identity

Article author: Arthur Isaev