---
url: /docs/guide/usage/linter/rules/vitest/require-mock-type-parameters.md
---

### What it does

Enforces the use of type parameters on `vi.fn()`, and optionally on `vi.importActual()` and `vi.importMock()`.

By default, only `vi.fn()` is checked. Set `checkImportFunctions` to `true` to also check `vi.importActual()` and `vi.importMock()`.

### Why is this bad?

Without explicit type parameters, `vi.fn()` creates a mock typed as `(...args: any[]) => any`.
This disables type checking between the mock and the real implementation, which can lead to two problems:

* tests that fail due to incorrect mock usage when they should pass
* or worse, tests that pass while the mock silently diverges from the actual runtime behavior.

### Examples

Examples of **incorrect** code for this rule configured as `{ "checkImportFunctions": false }`:

```ts
import { vi } from "vitest";

test("foo", () => {
  const myMockedFn = vi.fn();
});
```

Examples of **incorrect** code for this rule configured as `{ "checkImportFunctions": true }`:

```ts
import { vi } from "vitest";

vi.mock("./example.js", async () => {
  const originalModule = await vi.importActual("./example.js");

  return { ...originalModule };
});
const fs = await vi.importMock("fs");
```

Examples of **correct** code for this rule configured as `{ "checkImportFunctions": false }`:

```ts
import { vi } from "vitest";

test("foo", () => {
  const myMockedFnOne = vi.fn<(arg1: string, arg2: boolean) => number>();
  const myMockedFnTwo = vi.fn<() => void>();
  const myMockedFnThree = vi.fn<any>();
});
```

Examples of **correct** code for this rule configured as `{ "checkImportFunctions": true }`:

```ts
import { vi } from "vitest";

vi.mock("./example.js", async () => {
  const originalModule = await vi.importActual<any>("./example.js");

  return { ...originalModule };
});
const fs = await vi.importMock<any>("fs");
```

## Configuration

This rule accepts a configuration object with the following properties:

### checkImportFunctions

type: `boolean`

default: `false`

Also require type parameters for `importActual` and `importMock`.

## How to use

## Version

This rule was added in v1.58.0.

## References