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Answer: Mocking means creating a fake version of an internal or external service. Vitest exposes that work through the vi helper, which you import from vitest or use globally when global is enabled. If you are not familiar with vi.fn, vi.mock or vi.spyOn methods, the API section covers them.

Resetting and limits

Clear or restore mocks before or after each test so mock state does not leak between runs. A vi.mock call is hoisted to the top of the file and runs before imports. Some vi.spyOn patterns on module exports do not work in browser mode.

The spy helpers in source

fn wraps a function in a mock instance, and if you pass a value that is already a mock it returns that same function. isMockFunction is true when fn is a function, '_isMockFunction' is in fn, and fn._isMockFunction === true. spyOn requires a defined object as its first argument.

See also

• vitest-test-runner
• vitest-expect-assertions

Claims

• id: C1 claim: A fake stand-in for a service is commonly called mocking. source: S1 evidence: "This is commonly referred to as mocking."

• id: C2 claim: Vitest provides mocking utilities through its vi helper. source: S1 evidence: "Vitest provides utility functions to help you out through its vi helper."

• id: C3 claim: fn returns the same function if it is already a mock. source: S2 evidence: "if the function is already a mock, just return the same function,"

• id: C4 claim: isMockFunction is true when the function has the _isMockFunction flag. source: S2 evidence: "typeof fn === 'function' && '_isMockFunction' in fn && fn._isMockFunction === true"

Sources

• [S1] Mocking. Vitest docs. https://vitest.dev/guide/mocking . License: MIT. Accessed 2026-11-11.

• [S2] packages/spy. Vitest source. https://github.com/vitest-dev/vitest/blob/main/packages/spy/src/index.ts . License: MIT. Accessed 2026-11-11.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted). Sources and notices: Vitest documentation and source (vitest-dev/vitest): Copyright (c) 2021-Present VoidZero Inc. and Vitest contributors; MIT (https://raw.githubusercontent.com/vitest-dev/vitest/main/LICENSE). Text was condensed and changed from the originals.

@misc{iil_note_28d9g342,
  author = {{iinventedlight}},
  title = {{Vitest mocking builds fake stand-ins through the vi helper}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-11},
  url = {https://iinventedlight.com/library/notes/vitest-mocking/},
  note = {Permanent ID iil:note:28d9g342. License: CC-BY-4.0.},
}

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