func TestValidateEmail(t *testing.T) {
    tests := []struct {
        name    string
        email   string
        wantErr bool
    }{
        {
            name:    "valid email",
            email:   "user@example.com",
            wantErr: false,
        },
        {
            name:    "missing @",
            email:   "userexample.com",
            wantErr: true,
        },
        {
            name:    "empty string",
            email:   "",
            wantErr: true,
        },
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            err := ValidateEmail(tt.email)
            if (err != nil) != tt.wantErr {
                t.Errorf("ValidateEmail(%q) error = %v, wantErr %v", 
                    tt.email, err, tt.wantErr)
            }
        })
    }
}

Benefits:

Easy to add new test cases
Clear test case documentation
Parallel test execution with t.Parallel()
Isolated subtests with t.Run()

Test Helpers

Use t.Helper() to mark helper functions:

func assertNoError(t *testing.T, err error) {
    t.Helper()
    if err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
}

func assertEqual(t *testing.T, got, want interface{}) {
    t.Helper()
    if !reflect.DeepEqual(got, want) {
        t.Errorf("got %v, want %v", got, want)
    }
}

Benefits:

Correct line numbers in test failures
Reusable test utilities
Cleaner test code

Test Fixtures

Use t.Cleanup() for resource cleanup:

func testDB(t *testing.T) *sql.DB {
    t.Helper()
    
    db, err := sql.Open("sqlite3", ":memory:")
    if err != nil {
        t.Fatalf("failed to open test db: %v", err)
    }
    
    // Cleanup runs after test completes
    t.Cleanup(func() {
        if err := db.Close(); err != nil {
            t.Errorf("failed to close db: %v", err)
        }
    })
    
    return db
}

func TestUserRepository(t *testing.T) {
    db := testDB(t)
    repo := NewUserRepository(db)
    // ... test logic
}

Race Detection

Always run tests with the -race flag to detect data races:

go test -race ./...

In CI/CD:

- name: Test with race detector
  run: go test -race -timeout 5m ./...

Why:

Detects concurrent access bugs
Prevents production race conditions
Minimal performance overhead in tests

Coverage Analysis

Basic Coverage

go test -cover ./...

Detailed Coverage Report

go test -coverprofile=coverage.out ./...
go tool cover -html=coverage.out

Coverage Thresholds

# Fail if coverage below 80%
go test -cover ./... | grep -E 'coverage: [0-7][0-9]\.[0-9]%' && exit 1

Benchmarking

func BenchmarkValidateEmail(b *testing.B) {
    email := "user@example.com"
    
    b.ResetTimer()
    for i := 0; i < b.N; i++ {
        ValidateEmail(email)
    }
}

Run benchmarks:

go test -bench=. -benchmem

Compare benchmarks:

go test -bench=. -benchmem > old.txt
# make changes
go test -bench=. -benchmem > new.txt
benchstat old.txt new.txt

Mocking

Interface-Based Mocking

type UserRepository interface {
    GetUser(id string) (*User, error)
}

type mockUserRepository struct {
    users map[string]*User
    err   error
}

func (m *mockUserRepository) GetUser(id string) (*User, error) {
    if m.err != nil {
        return nil, m.err
    }
    return m.users[id], nil
}

func TestUserService(t *testing.T) {
    mock := &mockUserRepository{
        users: map[string]*User{
            "1": {ID: "1", Name: "Alice"},
        },
    }
    
    service := NewUserService(mock)
    // ... test logic
}

Integration Tests

Build Tags

//go:build integration
// +build integration

package user_test

func TestUserRepository_Integration(t *testing.T) {
    // ... integration test
}

Run integration tests:

go test -tags=integration ./...

Test Containers

func TestWithPostgres(t *testing.T) {
    if testing.Short() {
        t.Skip("skipping integration test")
    }
    
    // Setup test container
    ctx := context.Background()
    container, err := testcontainers.GenericContainer(ctx, ...)
    assertNoError(t, err)
    
    t.Cleanup(func() {
        container.Terminate(ctx)
    })
    
    // ... test logic
}

Test Organization

File Structure

package/
├── user.go
├── user_test.go          # Unit tests
├── user_integration_test.go  # Integration tests
└── testdata/             # Test fixtures
    └── users.json

Package Naming

// Black-box testing (external perspective)
package user_test

// White-box testing (internal access)
package user

Common Patterns

Testing HTTP Handlers

func TestUserHandler(t *testing.T) {
    req := httptest.NewRequest("GET", "/users/1", nil)
    rec := httptest.NewRecorder()
    
    handler := NewUserHandler(mockRepo)
    handler.ServeHTTP(rec, req)
    
    assertEqual(t, rec.Code, http.StatusOK)
}

Testing with Context

func TestWithTimeout(t *testing.T) {
    ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
    defer cancel()
    
    err := SlowOperation(ctx)
    if !errors.Is(err, context.DeadlineExceeded) {
        t.Errorf("expected timeout error, got %v", err)
    }
}

Best Practices

Use t.Parallel() for independent tests
Use testing.Short() to skip slow tests
Use t.TempDir() for temporary directories
Use t.Setenv() for environment variables
Avoid init() in test files
Keep tests focused - one behavior per test
Use meaningful test names - describe what's being tested

When to Use This Skill

Writing new Go tests
Improving test coverage
Setting up test infrastructure
Debugging flaky tests
Optimizing test performance
Implementing integration tests

golang-testing

Go Testing

Testing Framework

Table-Driven Tests