testing-strategy

Testing Strategy

Testing approach: unit/integration/e2e split, what to test, test structure (AAA pattern), mocking strategies, and coverage targets.

Testing Pyramid

Follow the testing pyramid — more unit tests, fewer integration tests, even fewer e2e tests:

        /  E2E  \           ~5%   — Critical user journeys
       /----------\
      / Integration \       ~20%  — Module boundaries, API contracts
     /----------------\
    /    Unit Tests     \   ~75%  — Functions, components, utilities
   /____________________\

Level	Speed	Scope	Quantity
Unit	Fast (ms)	Single function/component	Many
Integration	Medium (seconds)	Module interactions, DB, API	Some
E2E	Slow (seconds-minutes)	Full user flows	Few

What to Test

Always Test

• Business logic and calculations
• Data transformations and mappings
• Validation rules
• Error handling paths
• Edge cases: empty inputs, boundary values, null/undefined
• Public API of modules (exported functions)
• State transitions (reducers, state machines)
• Critical user journeys (e2e)

Don't Test

• Third-party library internals
• Simple getters/setters with no logic
• Framework boilerplate (constructor, lifecycle method existence)
• Implementation details (private methods, internal state shape)
• Constants and configuration values
• CSS styling (use visual regression tools instead)

Test Structure — AAA Pattern

Every test should follow Arrange, Act, Assert:

describe("OrderService", () => {
  describe("calculateTotal", () => {
    it("applies percentage discount to subtotal", () => {
      // Arrange
      const items = [
        { name: "Widget", price: 25.00, quantity: 2 },
        { name: "Gadget", price: 15.00, quantity: 1 },
      ];
      const discount = { type: "percentage", value: 10 };

      // Act
      const total = calculateTotal(items, discount);

      // Assert
      expect(total).toBe(58.50); // (50 + 15) * 0.90
    });
  });
});