Testing with Minitest

Writing tests for your code.

Testing with Minitest

Minitest is Ruby's built-in testing framework. It's fast, simple, and powerful. You don't need to install anything extra — it comes with Ruby. Minitest lets you write tests that verify your code works correctly.

require "minitest/autorun"

class TestCalculator < Minitest::Test
  def test_addition
    assert_equal 4, 2 + 2
  end

  def test_subtraction
    assert_equal 5, 10 - 5
  end

  def test_division
    assert_equal 3, 10 / 3  # integer division
  end
end

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Assert Methods

Minitest provides many assert methods for different checks. Use the most specific one for clear test failure messages. The assertion library is your best friend when debugging.

require "minitest/autorun"

class TestAssertions < Minitest::Test
  def test_equality
    assert_equal 4, 2 + 2
    assert_equal "hello", "hello"
  end

  def test_truthiness
    assert_nil nil
    assert_nil false
    assert_empty []
    assert_empty ""
  end

  def test_types
    assert_instance_of String, "hello"
    assert_kind_of Object, "hello"
    assert_respond_to "hello", :length
  end

  def test_errors
    assert_raises(ZeroDivisionError) { 1 / 0 }
    assert_raises(ArgumentError) { raise ArgumentError }
  end

  def test_refutation
    refute_equal 1, 2
    refute_nil "not nil"
    refute_empty [1, 2, 3]
  end
end

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Setup and Teardown

setup runs before each test. teardown runs after each test. Use them to prepare test data and clean up. This keeps your tests independent and repeatable.

require "minitest/autorun"

class TestUser < Minitest::Test
  def setup
    @user = User.new("Alice", "alice@example.com")
    @other = User.new("Bob", "bob@example.com")
  end

  def teardown
    @user = nil
    @other = nil
  end

  def test_name
    assert_equal "Alice", @user.name
  end

  def test_email
    assert_equal "alice@example.com", @user.email
  end

  def test_equality
    refute_equal @user, @other
  end
end

class User
  attr_reader :name, :email

  def initialize(name, email)
    @name = name
    @email = email
  end
end

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Test::Unit Style

Minitest supports two styles: spec style (describe/it) and test style (class/method). Both work equally well. Pick the one that feels natural to you.

require "minitest/autorun"

# Spec style
describe "String" do
  it "can be reversed" do
    "hello".reverse.must_equal "olleh"
  end

  it "has a length" do
    "hello".length.must_equal 5
  end

  it "can be upcased" do
    "hello".upcase.must_equal "HELLO"
  end
end

# Test style
class StringTest < Minitest::Test
  def test_reverse
    assert_equal "olleh", "hello".reverse
  end

  def test_length
    assert_equal 5, "hello".length
  end
end

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Why Minitest and RSpec

Minitest is fast and minimal — it does one thing well. RSpec is more expressive with a rich DSL. Minitest is great for simple projects. RSpec shines in large codebases with complex test scenarios. Both are excellent choices.

# Minitest: fast, simple, built-in
# RSpec: expressive, feature-rich, external

# Run Minitest
# ruby test/my_test.rb

# Run with rake
# rake test

# RSpec equivalent:
# describe Calculator do
#   it "adds numbers" do
#     expect(Calculator.add(2, 2)).to eq(4)
#   end
# end

# Minitest equivalent:
class CalculatorTest < Minitest::Test
  def test_add
    assert_equal 4, Calculator.add(2, 2)
  end
end

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🧪 Quick Quiz

What test framework comes with Ruby?

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