Test doubles concept in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby test double example?

Consider this Ruby code using RSpec test doubles. What will be printed?

Ruby

require 'rspec/mocks/standalone'

user = double('User')
allow(user).to receive(:name).and_return('Alice')

puts user.name

A Alice

B nil

C undefined method `name' for #<Double User>

D Error: NoMethodError

💡 Hint

Test doubles can be told to return specific values for methods.

The double user is told to return 'Alice' when name is called, so puts user.name prints 'Alice'.

🧠 Conceptual

intermediate

What is the main purpose of a test double in Ruby testing?

Choose the best description of why test doubles are used in Ruby tests.

A To speed up the Ruby interpreter during tests

B To replace real objects with simplified stand-ins to isolate tests

C To automatically generate test data for database records

D To log all method calls during test execution

💡 Hint

Think about how test doubles help focus tests on one part of the code.

Test doubles replace real objects so tests can focus on the code being tested without side effects or dependencies.

🔧 Debug

advanced

Why does this Ruby test double code raise an error?

Look at this Ruby code snippet using RSpec doubles. What causes the error?

Ruby

require 'rspec/mocks/standalone'

order = double('Order')

puts order.total

A Missing 'require' statement for RSpec doubles

B The double 'Order' is not created correctly without a block

C The 'puts' method cannot print double objects

D No method stub for 'total' was defined, so calling it raises an error

💡 Hint

Think about what happens when you call a method on a double without stubbing it.

By default, doubles do not respond to any methods unless explicitly stubbed. Calling total raises a NoMethodError.

📝 Syntax

advanced

Which Ruby code correctly creates a test double with a stubbed method?

Choose the option that correctly creates a test double named 'Car' with a method 'start' returning 'vroom'.

A car = double('Car').stub(:start).and_return('vroom')

B car = double('Car') { def start; 'vroom'; end }

C car = double('Car', start: 'vroom')

D car = double('Car').allow(:start).to_return('vroom')

💡 Hint

Look for the correct syntax to stub a method when creating a double.

Option C uses the correct syntax to create a double with a stubbed method. Options B, C, and D have syntax errors or invalid method chains.

🚀 Application

expert

How many methods will this Ruby test double respond to?

Given this Ruby code, how many methods will the user double respond to?

Ruby

require 'rspec/mocks/standalone'

user = double('User')
allow(user).to receive(:name).and_return('Bob')
allow(user).to receive(:email).and_return('[e-mail]')

methods = user.methods.select { |m| [:name, :email].include?(m) }
puts methods.size

A 2

B 0

C All methods of Object plus :name and :email

D Raises an error because methods is undefined

💡 Hint

Consider only the methods explicitly stubbed on the double.

The double responds only to the stubbed methods :name and :email, so the count is 2.