Software Testing — Types ও Test Pyramid

Testing কেন?

• Bug early catch — cost 100x কম
• Refactor confidently
• Documentation হিসেবে কাজ করে
• Regression prevent

Test Pyramid ⭐

        /\
       /E2E\       Few (slow, expensive)
      /------\
     /Integra-\    Some
    / tion     \
   /------------\
  / Unit Tests  \  Many (fast, cheap)
 /--------------\

Types

Type	Scope
Unit	1 function/class (mock deps)
Integration	Multiple components (real DB)
Contract	API shape (consumer/provider)
E2E	Full user flow (browser)
Smoke	Critical paths post-deploy
Regression	Old bugs don't return
Acceptance (UAT)	Business validates

Unit Test — AAA Pattern

[Fact]
public void Add_Two_Numbers() {
  // Arrange
  var calc = new Calculator();
  // Act
  var result = calc.Add(2, 3);
  // Assert
  Assert.Equal(5, result);
}

Test Naming

MethodUnderTest_Scenario_ExpectedBehavior

Withdraw_InsufficientBalance_ThrowsException
Login_ValidCredentials_ReturnsToken
CalculateDiscount_VIPCustomer_Returns20Percent

Mocking

// Moq (.NET)
var repo = new Mock<IUserRepository>();
repo.Setup(r => r.GetById(1))
    .ReturnsAsync(new User { Id = 1, Name = "Rahim" });

var service = new UserService(repo.Object);
var user = await service.GetUserAsync(1);
Assert.Equal("Rahim", user.Name);
repo.Verify(r => r.GetById(1), Times.Once);

TDD (Test-Driven Development) ⭐

Red   — failing test লিখুন
  ↓
Green — pass করার minimum code
  ↓
Refactor — clean up (tests green রাখুন)
  ↓ repeat

BDD (Behavior-Driven) ⭐

Feature: User Login
  Scenario: Valid credentials
    Given a registered user with email "rahim@x.com"
    When they login with correct password
    Then they should see the dashboard
    And receive a JWT token

Tools: Cucumber, SpecFlow (.NET), Behat (PHP)।

Code Coverage

• Line coverage — which lines run
• Branch coverage — if/else both sides
• Target: 70-80% sensible
• ❌ 100% target counterproductive
• Tools: Jest, coverlet, JaCoCo, Istanbul

Test Doubles

• Dummy — placeholder, not used
• Stub — fake return value
• Spy — record calls
• Mock — verify behavior
• Fake — working simpler impl (in-memory DB)

Testing Frameworks

xUnit/NUnit (.NET), Jest/Vitest (JS), pytest (Python), JUnit (Java), PHPUnit (PHP), RSpec (Ruby)।

Best Practices

• Fast (< 1s unit)
• Isolated (no shared state)
• Repeatable (same input = same output)
• Self-validating (pass/fail, no manual check)
• Timely (write with code, not after)
• One assertion concept per test
• Test behavior, not implementation