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.

• ID: finserv-form-validation.error-edge-cases.calculation-testing

• Severity: info

• What to look for: Count all financial calculation functions (interest, fees, tax, rounding, conversions). For each, check whether a corresponding test file exists in __tests__/, test/, or alongside as .test.ts/.spec.ts. Count the test cases and classify each as happy-path or edge-case (zero amounts, min/max limits, rounding boundaries like $0.005, negative inputs). Report: "X of Y financial functions have tests; Z edge-case scenarios covered."

• Pass criteria: At least 100% of financial calculation functions have unit tests. Tests include at least 3 edge-case categories: zero amounts, min/max boundary values, and rounding boundaries (e.g., $10.005). No more than 0 financial functions lack test coverage entirely.

• Fail criteria: Any financial calculation function has no test file, or tests cover only happy-path scenarios with fewer than 3 edge-case categories.

• Skip (N/A) when: The project has no financial calculations (0 financial calculation functions found), or calculations are trivial (single multiplication with no rounding).

• Detail on fail: Example: "No tests found for interest calculations. Only happy-path scenarios tested, no edge cases like $0.005 rounding."

• Cross-reference: The no-float-accumulation check verifies integer arithmetic at runtime; this check verifies those patterns are tested to prevent regressions.

• Remediation: Write comprehensive tests in src/lib/finance.test.ts:

  In src/lib/finance.test.ts:

  import { calculateInterest, bankersRound } from './finance';
  
  describe('calculateInterest', () => {
    it('should handle zero principal', () => {
      expect(calculateInterest(0, 0.05)).toBe(0);
    });
  
    it('should round correctly at 0.005', () => {
      expect(bankersRound(10000.5)).toBe(10001);
    });
  
    it('should handle maximum amounts', () => {
      const max = 999999; // $9,999.99
      expect(calculateInterest(max, 0.05)).toBeLessThanOrEqual(max);
    });
  
    it('should not accumulate floating-point errors', () => {
      let balance = 100000;
      for (let i = 0; i < 100; i++) {
        balance = Math.round(balance * 1.001);
      }
      expect(balance % 1).toBe(0);
    });
  });