vitest/prefer-spy-on

Date.now = vi.fn() throws the real Date.now away. Unless you save it and put it back yourself, every test that runs afterwards gets the mock. vi.spyOn(Date, "now") remembers the original, so mockRestore() or vi.restoreAllMocks() brings it back.

--fix-suggestions rewrites obj.save = vi.fn() to vi.spyOn(obj, 'save').mockImplementation(). The .mockImplementation() is there because a spy calls the original function until you give it an implementation.

Configure

Off by default. To turn it on:

[lint.rules]
"vitest/prefer-spy-on" = "error" # or "warn"

Examples

Reported:

import { beforeEach, vi } from "vitest";

beforeEach(() => {
  Date.now = vi.fn(() => 1700000000000);
  console.warn = vi.fn();
});

Not reported:

import { afterEach, beforeEach, vi } from "vitest";

beforeEach(() => {
  vi.spyOn(Date, "now").mockImplementation(() => 1700000000000);
  vi.spyOn(console, "warn").mockImplementation(() => {});
});

afterEach(() => {
  vi.restoreAllMocks();
});

A mock that's stored in a variable or written in an object literal doesn't replace anything, so it isn't reported:

import { vi } from "vitest";

const onSave = vi.fn();
const logger = { warn: vi.fn() };