jest/prefer-jest-mocked

fetchUser as jest.Mock replaces the function's type with a mock of anything, so mockReturnValue accepts a value that the real function can never return. jest.mocked(fetchUser) keeps the parameters and the return type, so a mock value with the wrong shape is a type error.

The rule reports a cast to jest.Mock, jest.MockedFunction, jest.MockedClass or jest.MockedObject, with or without type arguments.

--fix rewrites the cast to jest.mocked().

Configure

Off by default. To turn it on:

[lint.rules]
"jest/prefer-jest-mocked" = "error" # or "warn"

Examples

Reported:

import { jest } from "@jest/globals";
import { fetchUser, saveUser } from "./api";

jest.mock("./api");

(fetchUser as jest.Mock).mockReturnValue({ name: "Ada" });
const mockSaveUser = saveUser as jest.MockedFunction<typeof saveUser>;

Not reported:

import { jest } from "@jest/globals";
import { fetchUser, saveUser } from "./api";

jest.mock("./api");

jest.mocked(fetchUser).mockReturnValue({ name: "Ada" });
const mockSaveUser = jest.mocked(saveUser);