How would you implement Test-Driven Development (TDD) for a real feature — describe the workflow

TDD Cycle: Red → Green → Refactor

1. 🔴 RED: Write a failing test
2. 🟢 GREEN: Write minimal code to pass
3. 🔵 REFACTOR: Clean up while tests pass
4. Repeat

Feature: Deadline Reminders

Step 1: RED — Test for finding upcoming deadlines

// deadline-reminder.test.ts
describe("getUpcomingDeadlines", () => {
  it("returns applications due within 24 hours", () => {
    const now = new Date();
    const applications = [
      { id: "1", deadline: new Date(now.getTime() + 12 * 3600000) }, // 12h away
      { id: "2", deadline: new Date(now.getTime() + 48 * 3600000) }, // 48h away
      { id: "3", deadline: new Date(now.getTime() - 1 * 3600000) },  // Past
    ];

    const result = getUpcomingDeadlines(applications, now);
    expect(result).toHaveLength(1);
    expect(result[0].id).toBe("1");
  });
});
// Run test → FAILS (getUpcomingDeadlines doesn't exist)

Step 2: GREEN — Minimal implementation

// deadline-reminder.ts
export function getUpcomingDeadlines(applications: App[], now: Date): App[] {
  const in24h = new Date(now.getTime() + 24 * 3600000);
  return applications.filter(app => app.deadline > now && app.deadline <= in24h);
}
// Run test → PASSES ✅

Step 3: REFACTOR (if needed)

// Clean up variable names, add types
export function getUpcomingDeadlines(apps: Application[], referenceDate: Date): Application[] {
  const twentyFourHoursFromNow = new Date(referenceDate.getTime() + 24 * HOUR_MS);
  return apps.filter(({ deadline }) =>
    deadline > referenceDate && deadline <= twentyFourHoursFromNow
  );
}
// Tests still pass ✅

Step 4: Repeat — Add email sending test

it("sends reminder email for each upcoming deadline", async () => {
  const apps = [{ id: "1", deadline: in12Hours, userId: "u1", company: "Google" }];
  const mockSend = vi.fn().mockResolvedValue(undefined);

  await sendDeadlineReminders(apps, mockSend);

  expect(mockSend).toHaveBeenCalledTimes(1);
  expect(mockSend).toHaveBeenCalledWith(
    expect.objectContaining({ subject: expect.stringContaining("Google") })
  );
});

TDD Benefits

BenefitWhy

Design feedbackIf test is hard to write, code needs redesign
ConfidenceRefactor fearlessly with tests as safety net
DocumentationTests describe expected behavior
Fewer bugsCatch bugs before code reaches main
Better APIsYou're the first consumer of your API