Test it with Vitest

Goal

Run, read and write tests, and know what types do and do not catch.

Run the tests

npx vitest run streak     # once
npx vitest streak         # watch mode: re-runs as you save

Vitest runs TypeScript directly; no build step needed.

Types are not tests

The checker knows currentStreak returns a number. It cannot know that the number is right. That is what tests are for. From streak/test/habits.test.ts:

it('counts days in a row, ending today or yesterday', () => {
  const days: Day[] = ['2026-10-14', '2026-10-15', '2026-10-16']
  expect(currentStreak(habit({ checkIns: days }), TODAY)).toBe(3)
  expect(currentStreak(habit({ checkIns: [...days, TODAY] }), TODAY)).toBe(4)
  expect(currentStreak(habit({ checkIns: ['2026-10-15'] }), TODAY)).toBe(0)
})

Design that makes testing easy

Three choices from earlier chapters pay off here:

1. Pure functions. addHabit, checkIn and currentStreak take data and return data.
2. Today is a parameter. Tests pass a fixed TODAY, so they never break at midnight.
3. The CLI takes a Store. Tests give it memoryStore(), no files involved. From streak/test/cli.test.ts:

const store = memoryStore()

expect(await run(['add', 'Read', '--emoji', '📚'], store, TODAY)).toEqual({
  code: 0,
  output: 'Added 📚 Read (id: read)',
})

Typed test helpers

A small factory keeps tests short and still type-checked:

function habit(overrides: Partial<Habit> = {}): Habit {
  return {
    id: 'read',
    name: 'Read',
    emoji: '📚',
    frequency: { kind: 'daily' },
    createdOn: '2026-10-01',
    checkIns: [],
    ...overrides,
  }
}

Partial<Habit> is a built-in generic: every field of Habit, all optional.

Check it works

Break currentStreak on purpose (start streak at 1). Run the tests and read the failure. Undo it.

Exercise

Write a test for a weekly habit that missed last week entirely: its streak should be 0 unless this week already reached the target.