Question

Can anybody tell me why a lot of Ruby boolean methods use this double negation convention?

!!(boolean expression)

Answer 1

The double negation ensures that no matter the initial value, you will always get true or false, never some mystery value.

This is handy because it avoids dangling references to objects you no longer require, or having to differentiate between two types of false value, nil and false.

Often you will see methods written like this:

def logged_in?
  !!@session_user
end

This will return true or false and that value can be used for whatever purpose is required. Compare that with this:

def logged_in?
  @session_user
end

In this case if you save the value, you’re actually saving the whole @session_user object, which could be a fairly significant chunk of memory. This memory cannot be released until your reference to it falls out of scope. Since there is only one true and one false, there’s no need for garbage collection.