Test files present and organized

A codebase with no test files has no automated verification that the core logic works correctly after a change. Every refactor, dependency upgrade, or new feature becomes a manual regression risk. ISO 25010:2011 §6.5.5 identifies absence of test infrastructure as a reliability defect. For AI-built projects specifically, the lack of tests is a compounding risk: the original author may not remember the edge cases, and future changes can silently break behavior that was never encoded as an assertion.

Set up a test framework appropriate for your stack. For Next.js/React with TypeScript, Vitest + Testing Library is the standard starting point.

npm install -D vitest @testing-library/react @testing-library/user-event jsdom

Add to package.json:

{
  "scripts": {
    "test": "vitest",
    "test:coverage": "vitest run --coverage"
  }
}

Create your first test co-located with the module it covers:

// src/lib/format-price.test.ts
import { describe, it, expect } from 'vitest'
import { formatPrice } from './format-price'

describe('formatPrice', () => {
  it('formats a whole dollar amount', () => {
    expect(formatPrice(1000)).toBe('$10.00')
  })
  it('returns "Free" for zero', () => {
    expect(formatPrice(0)).toBe('Free')
  })
})