vitest/require-mock-type-parameters

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.

Configure

Off by default. To turn it on:

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

Every option, set to its default:

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

Examples

Reported:

import { vi } from "vitest";

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

Not reported:

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().

import { vi } from "vitest";

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

import { vi } from "vitest";

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