Coverage Strategist

Define pragmatic, ROI-focused test coverage strategies.

Coverage Philosophy

Goal: Maximum confidence with minimum tests

Principle: 100% coverage is not the goal. Test what matters.

Critical Path Identification

// Critical paths that MUST be tested const criticalPaths = { authentication: { priority: "P0", coverage: "100%", paths: [ "User login flow", "User registration", "Password reset", "Token refresh", "Session management", ], reasoning: "Security critical, impacts all users", },

checkout: { priority: "P0", coverage: "100%", paths: [ "Add to cart", "Update cart", "Apply coupon", "Process payment", "Order confirmation", ], reasoning: "Revenue critical, business essential", },

dataIntegrity: { priority: "P0", coverage: "100%", paths: [ "User data CRUD", "Order creation", "Inventory updates", "Database transactions", ], reasoning: "Data corruption would be catastrophic", }, };

// Important but not critical const importantPaths = { userProfile: { priority: "P1", coverage: "80%", paths: ["Profile updates", "Avatar upload", "Preferences"], reasoning: "Important UX, but not business critical", },

search: { priority: "P1", coverage: "70%", paths: ["Product search", "Filters", "Sorting"], reasoning: "Enhances experience, not essential", }, };

Layer-Specific Targets

Coverage Targets by Layer

Business Logic / Core Functions: 90-100%

Why: High ROI - complex logic, many edge cases What to test:

• Calculations
• Validations
• State machines
• Algorithms
• Data transformations

API Endpoints: 80-90%

Why: Critical integration points What to test:

• Happy paths
• Error cases
• Validation
• Authentication
• Authorization

Database Layer: 70-80%

Why: Data integrity matters What to test:

• CRUD operations
• Transactions
• Constraints
• Migrations

UI Components: 50-70%

Why: Lower ROI - visual changes, less critical What to test:

• User interactions
• State changes
• Error states
• Critical flows only

Utils/Helpers: 80-90%

Why: Reused everywhere, high impact What to test:

• All public functions
• Edge cases
• Error handling

"Don't Test This" List

// Explicit list of what NOT to test

const dontTestThese = { externalLibraries: { examples: ["React internals", "Next.js router", "Lodash functions"], reasoning: "Already tested by library authors", },

trivialCode: { examples: [ "Simple getters/setters", "Constants", "Type definitions", "Pass-through functions", ], reasoning: "No logic to test, waste of time", },

presentationalComponents: { examples: ["Simple buttons", "Icons", "Layout wrappers"], reasoning: "Visual regression testing more appropriate", },

configurationFiles: { examples: ["webpack.config.js", "next.config.js"], reasoning: "Configuration, not logic", },

mockData: { examples: ["Fixtures", "Test data", "Storybook stories"], reasoning: "Not production code", }, };

// Example: Don't test trivial code // ❌ Don't test this class User { constructor(private name: string) {} getName() { return this.name; } // Trivial getter }

// ✅ But DO test this class User { constructor(private name: string) {}

getDisplayName() { // Business logic return this.name .split(" ") .map((n) => n.charAt(0).toUpperCase() + n.slice(1)) .join(" "); } }

Test Priority Matrix

interface TestPriority { feature: string; businessImpact: "high" | "medium" | "low"; complexity: "high" | "medium" | "low"; changeFrequency: "high" | "medium" | "low"; priority: "P0" | "P1" | "P2" | "P3"; targetCoverage: string; }

const testPriorities: TestPriority[] = [ { feature: "Payment processing", businessImpact: "high", complexity: "high", changeFrequency: "low", priority: "P0", targetCoverage: "100%", }, { feature: "User authentication", businessImpact: "high", complexity: "medium", changeFrequency: "low", priority: "P0", targetCoverage: "100%", }, { feature: "Product search", businessImpact: "medium", complexity: "medium", changeFrequency: "medium", priority: "P1", targetCoverage: "80%", }, { feature: "UI themes", businessImpact: "low", complexity: "low", changeFrequency: "high", priority: "P3", targetCoverage: "30%", }, ];

// Priority calculation function calculatePriority( businessImpact: number, // 1-10 complexity: number, // 1-10 changeFrequency: number // 1-10 ): number { return businessImpact * 0.5 + complexity * 0.3 + changeFrequency * 0.2; }

Coverage Configuration

// jest.config.js module.exports = { collectCoverageFrom: [ "src//*.{ts,tsx}", "!src//.d.ts", "!src/**/.stories.tsx", // Don't count stories "!src/mocks/", // Don't count mocks "!src//tests/**", // Don't count tests ],

coverageThresholds: { global: { statements: 70, branches: 65, functions: 70, lines: 70, }, // Critical paths: 90%+ "./src/services/payment//*.ts": { statements: 90, branches: 85, functions: 90, lines: 90, }, "./src/services/auth//.ts": { statements: 90, branches: 85, functions: 90, lines: 90, }, // Utils: 80%+ "./src/utils/**/.ts": { statements: 80, branches: 75, functions: 80, lines: 80, }, // UI components: 50%+ (lower bar) "./src/components/**/*.tsx": { statements: 50, branches: 45, functions: 50, lines: 50, }, }, };

Test Investment ROI

// Calculate ROI of testing interface TestROI { feature: string; testingCost: number; // hours bugPreventionValue: number; // estimated $ saved roi: number; // ratio }

const testROI: TestROI[] = [ { feature: "Payment processing", testingCost: 40, // hours bugPreventionValue: 50000, // Could lose $50k revenue roi: 1250, // $1,250 per hour invested }, { feature: "Authentication", testingCost: 20, bugPreventionValue: 10000, // Security breach cost roi: 500, }, { feature: "Theme switcher", testingCost: 5, bugPreventionValue: 100, // Minor UX issue roi: 20, }, ];

// Focus on high ROI tests const sortedByROI = testROI.sort((a, b) => b.roi - a.roi);

Pragmatic Testing Strategy

Testing Strategy Document

Principles

1. Business value first: Test what breaks the business
2. Edge cases over happy path: Happy path is obvious
3. Integration over unit: Test how pieces work together
4. Critical flows end-to-end: User journeys matter most

Test Types Distribution

• 70% Unit tests (fast, isolated)
• 20% Integration tests (API + DB)
• 10% E2E tests (critical flows only)

Coverage Goals

• Overall: 70% (pragmatic goal)
• Critical business logic: 90%+
• API endpoints: 80%+
• UI components: 50%+ (user interactions only)

What NOT to Test

• Third-party libraries
• Trivial getters/setters
• Pure presentational components
• Configuration files
• Mock data and fixtures

Review Criteria

Before writing a test, ask:

1. What bug would this test prevent?
2. How likely is that bug?
3. How costly would that bug be?
4. Is this already covered by integration tests?

If ROI is low, skip the test.

Team Guidelines

// Code review checklist for test coverage

const reviewChecklist = { criticalPath: { question: "Does this change affect a critical path?", ifYes: "MUST have comprehensive tests (90%+)", },

businessLogic: { question: "Is this complex business logic?", ifYes: "MUST have unit tests with edge cases", },

apiEndpoint: { question: "Is this a new API endpoint?", ifYes: "MUST have integration tests", },

uiComponent: { question: "Is this a UI component?", ifYes: "Optional - test interactions only", },

bugFix: { question: "Is this a bug fix?", ifYes: "MUST have regression test", }, };

Best Practices

• Focus on risk: Test what could go wrong

• Diminishing returns: 100% coverage has low ROI

• Integration over unit: Test behavior, not implementation

• Critical paths first: Payment, auth, data integrity

• Explicit "don't test": Be intentional about skipping

• Review regularly: Adjust targets quarterly

• Measure bugs: Track if tests catch real issues

Output Checklist

• Critical paths identified

• Layer-specific targets defined

• "Don't test this" list created

• Priority matrix established

• Coverage thresholds configured

• ROI analysis performed

• Testing strategy documented

• Team guidelines defined