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SAF

•Created by SAF on 4/30/2025 in #help-and-questions

What's the purpose of this unit test?

I'm looking into testing database operations so I'm reading this blog post on unit testing. The first unit test written is this:

// src/quotes/tags.service.test.ts
describe('tags.service', () => {
  beforeEach(() => {
    vi.restoreAllMocks()
  })
  describe('upsertTags', () => {
    it('should return a list of tagIds', async () => {
      prismaMock.$transaction.mockResolvedValueOnce([1, 2, 3])
      const tagIds = await TagService.upsertTags(['tag1', 'tag2', 'tag3'])
      expect(tagIds).toStrictEqual([1, 2, 3])
    })
  })
})

// src/quotes/tags.service.test.ts
describe('tags.service', () => {
  beforeEach(() => {
    vi.restoreAllMocks()
  })
  describe('upsertTags', () => {
    it('should return a list of tagIds', async () => {
      prismaMock.$transaction.mockResolvedValueOnce([1, 2, 3])
      const tagIds = await TagService.upsertTags(['tag1', 'tag2', 'tag3'])
      expect(tagIds).toStrictEqual([1, 2, 3])
    })
  })
})

which tests this function:

// src/quotes/tags.service.ts
// ...
export const upsertTags = async (tags: string[]) => {
  return await prisma.$transaction(async tx => {
    // ...
  })
}

// src/quotes/tags.service.ts
// ...
export const upsertTags = async (tags: string[]) => {
  return await prisma.$transaction(async tx => {
    // ...
  })
}

But I don't get what this is supposed to test. upsertTags is one big transaction. So when you mock a transaction's return value and later test the mock, how does that test the behavior of the function in any way?

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