Use the double negation syntax to check for truthiness of values. All values correspond to a boolean, irrespective of their type.

irb(main):001:0> !!1234
=> true
irb(main):002:0> !!"Hello, world!"
(irb):2: warning: string literal in condition
=> true
irb(main):003:0> !!true
=> true
irb(main):005:0> !!{a:'b'}
=> true

All values except nil and false are truthy.

irb(main):006:0> !!nil
=> false
irb(main):007:0> !!false
=> false

You do not need to use double negation in if-else statements.

if 'hello'
    puts 'hey!'
else
    puts 'bye!'
end

The above code prints 'hey!' on the screen.

Remarks

As a rule of thumb, avoid using double-negations in code. Rubocop says (opens new window) that double negations are unnecessarily complex and can often be replaced with something more readable.

Instead of writing

def user_exists?
    !!user
end

use

def user_exists?
    !user.nil?
end