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docs: document the assertion function limitation in in-source testing (#10527)
authored by
koutaro-masaki
and committed by
GitHub
(Jun 7, 2026, 10:53 AM +0200)
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docs/api/assert.md
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docs/api/assert.md
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Vitest reexports the `assert` method from [`chai`](https://www.chaijs.com/api/assert/) for verifying invariants.
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::: warning In-Source Testing {#in-source-testing}
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When using [assertion functions](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions) such as `assert` from `import.meta.vitest` in [in-source tests](/guide/in-source), TypeScript reports error `TS2775` because they must be called via an explicitly annotated name. Annotate the variable with `Chai.Assert` or call it directly:
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::: code-group
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```ts [Annotated variable]
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if (import.meta.vitest) {
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const { test, assert } = import.meta.vitest // [!code --]
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const { test } = import.meta.vitest // [!code ++]
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const assert: Chai.Assert = import.meta.vitest.assert // [!code ++]
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test('assert', () => {
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assert('foo' !== 'bar', 'foo should not be equal to bar')
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})
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}
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```
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```ts [Direct call]
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if (import.meta.vitest) {
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const { test, assert } = import.meta.vitest // [!code --]
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const { test } = import.meta.vitest // [!code ++]
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test('assert', () => {
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assert('foo' !== 'bar', 'foo should not be equal to bar') // [!code --]
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import.meta.vitest!.assert('foo' !== 'bar', 'foo should not be equal to bar') // [!code ++]
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})
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}
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```
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:::
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## assert
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- **Type:** `(expression: any, message?: string) => asserts expression`
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docs/guide/in-source.md
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docs/guide/in-source.md
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Reference to [`examples/in-source-test`](https://github.com/vitest-dev/vitest/tree/main/examples/in-source-test) for the full example.
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::: warning
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There is a limitation when using [assertion functions](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions) such as `assert` in in-source tests. See [`assert`](/api/assert#in-source-testing) for details and workarounds.
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:::
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## Notes
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This feature could be useful for: