Hash#compact removes all keys with nil as value.

compact.gemspec

Gem::Specification.new do |s|
  s.name        = 'compact'
  s.version     = '0.0.1'
  s.platform    = Gem::Platform::RUBY
  s.author      = 'Stephan Schubert'
  s.email       = '[email protected]'
  s.summary     = 'Hash#compact'
  s.description = 'Removes all keys with nil as value.'
 
  s.files         = ['compact.rb']
  s.test_file     = 'compact_spec.rb'
  s.require_path  = '.'
 
  s.add_development_dependency('rspec', ["~> 2.0"])
end

compact.rb

module Compact
  def compact
    delete_if { |k, v| v.nil? }
  end
end

Hash.send :include, Compact

compact_spec.rb

require File.expand_path('compact')

describe Compact do
  let(:hash) { { a: nil, b: [5], c: "foo" } }

  describe "#compact" do
    subject { hash.compact }
    it      { should eq(b: [5], c: "foo") }
  end
end

Instead of

{ a: 3, b: (5 if false) } #=> { a: 3, b: nil }

or

{ a:3 }.tap { |h| h[:b] = 5 if false } #=> { a:3 }

you can just use

{ a:3, b: (5 if false) }.compact #=> { a: 3 }