# vitest/require-mock-type-parameters

> Require a type parameter on `vi.fn()`

A plain `vi.fn()` is typed as a function that takes any arguments and returns `any`. TypeScript then accepts `mockReturnValue(42)` on a mock of a function that returns a string. With a type parameter, TypeScript checks what you pass to the mock and what it returns.

A mock with an implementation, such as `vi.fn(() => 1)`, is reported too, even though TypeScript infers its type. The rule only checks TypeScript files.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

Off by default. To turn it on:

```toml bunfig.toml
[lint.rules]
"vitest/require-mock-type-parameters" = "error" # or "warn"
```

Every option, set to its default:

```toml bunfig.toml
[lint.rules]
"vitest/require-mock-type-parameters" = ["error", {
  checkImportFunctions = false,
}]
```

{/* End of the generated section. */}

## Examples

Reported:

{/* bad {"count": 2} */}
```ts
import { vi } from "vitest";

const findUser = vi.fn();
const log = vi.fn((message: string) => {});
```

Not reported:

{/* good */}
```ts
import { vi } from "vitest";

type User = { name: string };

const findUser = vi.fn<(id: string) => Promise<User>>();
const log = vi.fn<(message: string) => void>((message) => {});
```

## Options

| Option | Default | What it does |
| --- | --- | --- |
| `checkImportFunctions` | `false` | Also require a type parameter on `vi.importActual()` and `vi.importMock()`. |

{/* bad {"options": [{"checkImportFunctions": true}]} */}
```ts
import { vi } from "vitest";

const mailer = await vi.importActual("./mailer");
```

{/* good {"options": [{"checkImportFunctions": true}]} */}
```ts
import { vi } from "vitest";

const mailer = await vi.importActual<typeof import("./mailer")>("./mailer");
```