Quick test of RSpec's Instance double

double_spec.rb

RSpec.configure do |config|
  config.mock_with :rspec do |mocks|
    mocks.verify_doubled_constant_names = true
  end
end

class Greeting
  def greet
    'Hi'
  end
end

RSpec.describe Greeting do
  it 'can mock existing methods' do
    greeting = instance_double(Greeting)
    allow(greeting).to receive(:greet).and_return('Bye')

    expect(greeting.greet).to eq('Bye')
  end

  it 'can mock existing methods' do
    greeting = instance_double(Greeting)
    allow(greeting).to receive(:foo).and_return('Bye')

    expect(greeting.foo).to eq('Bye')
  end
end

This page has more info: https://relishapp.com/rspec/rspec-mocks/docs/verifying-doubles/using-an-instance-double

What I'm showing here is that you can mock existing methods only. So the Greeting class has a greet method, but not a foo method, so the first test passes, but the second doesn't.