Calculation logic unit-tested with edge cases

Financial calculation functions — rounding, interest accrual, fee application — are high-stakes and frequently modified. Without unit tests that cover zero amounts, min/max boundary values, and rounding-boundary inputs like $10.005, a refactor that silently changes behavior at those boundaries will ship undetected. ISO 25010 maintainability requires that critical business logic be verifiable without running the full application. Missing edge-case coverage is the most common root cause of financial bugs found only in production, often after incorrect charges have been issued.

Create src/lib/finance.test.ts (or equivalent) and include at minimum three edge-case categories: zero inputs, boundary amounts, and rounding midpoints.

// src/lib/finance.test.ts
import { calculateInterest, roundInterestCents } from './finance';

describe('calculateInterest', () => {
  it('returns 0 for zero principal', () =>
    expect(calculateInterest(0, 0.05)).toBe(0));

  it('handles maximum allowed amount', () =>
    expect(calculateInterest(99999, 0.05)).toBeGreaterThan(0));

  it('applies banker rounding at 0.5 cent boundary', () =>
    expect(roundInterestCents(100.5)).toBe(100)); // rounds to even

  it('produces integer output', () =>
    expect(Number.isInteger(roundInterestCents(10.335))).toBe(true));
});

Run tests in CI via npm test and gate merges on the test step passing.