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Answer: expect is used to create assertions. Vitest ships Chai assertions by default and Jest-compatible assertions on top of Chai. You can write expect(input).to.equal(2) or expect(input).toBe(2). Unlike Jest, Vitest also accepts a message as the second argument of expect; if the assertion fails, that message is the error text.

Types and extras

Since Vitest 4.1, spy and mock checks also have Chai-style forms such as expect(spy).to.have.been.called(), alongside Jest-style toHaveBeenCalled(). expect does not test TypeScript types by itself; use expectTypeOf or assertType for that. expect.assert re-exports Chai's assert API, which can narrow types in a way the to* matchers do not.

The matcher plugin in source

JestChaiExpect is a ChaiPlugin. It uses addMethod on the Assertion prototype and on the matchers object.

See also

• vitest-test-runner
• vitest-mocking

Claims

• id: C1 claim: expect is used to create assertions. source: S1 evidence: "expect is used to create assertions."

• id: C2 claim: Vitest ships Chai assertions by default. source: S1 evidence: "Vitest provides chai assertions by default"

• id: C3 claim: Jest-style expect matchers are a Chai plugin named JestChaiExpect. source: S2 evidence: "export const JestChaiExpect: ChaiPlugin = (chai, utils) => {"

• id: C4 claim: Those matchers are added on chai.Assertion.prototype. source: S2 evidence: "utils.addMethod(chai.Assertion.prototype, n, softWrapper)"

Sources

• [S1] expect. Vitest docs. https://vitest.dev/api/expect . License: MIT. Accessed 2026-11-11.

• [S2] JestChaiExpect matcher plugin. Vitest source. https://github.com/vitest-dev/vitest/blob/main/packages/expect/src/jest-expect.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_75fe3bry,
  author = {{iinventedlight}},
  title = {{Vitest expect creates assertions with both Chai and Jest APIs}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-11},
  url = {https://iinventedlight.com/library/notes/vitest-expect-assertions/},
  note = {Permanent ID iil:note:75fe3bry. License: CC-BY-4.0.},
}

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