Returns a hash with non nil values.

hash = { a: true, b: false, c: nil }
hash.compact        # => { a: true, b: false }
hash                # => { a: true, b: false, c: nil }
{ c: nil }.compact  # => {}
{ c: true }.compact # => { c: true }

Source: Hash

A little like Array#compact. Handy!