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Truthiness (only nil and false are falsy)

In Ruby, only nil and false count as no, and everything else counts as yes, even zero. It is like a door that opens for everyone except two special guests.

In this page:

  1. Truthiness (only nil and false are falsy)
Syntax
ruby 
if value
  # runs for anything except nil and false
end

Truthiness (only nil and false are falsy)

In Ruby, every value is truthy in a boolean context except nil and false -- notably, unlike many other languages, 0 and "" (empty string) are both truthy.

This is a frequent surprise for developers coming from languages where 0 or empty collections are falsy. Knowing this rule precisely avoids subtle conditional bugs.

Warning: Unlike many other languages, 0 and "" (empty string) are truthy in Ruby -- only nil and false are falsy.

Example: Truthiness (only nil and false are falsy)

markup 
puts "0 is truthy" if 0
puts "empty string is truthy" if ""
puts "this won't print" if nil
puts "this won't print either" if false
Related Topics

nil · Symbol vs String · method_missing · Frozen String Literals

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Common Mistakes
  1. Expecting 0 to be false, as in C or JavaScript, when it is truthy in Ruby.
  2. Expecting an empty string or array to be falsy; use .empty?.
  3. Using if x to check for false and nil separately, when both are falsy and x.nil? tests only nil.
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