Jest / Vitest assertions

Updated 2026-02-15

expect(), matchers, and async tests. toBe, toEqual, toThrow, and mocking basics.

expect matchers

expect(x).toBe(y) strict equal. expect(x).toEqual(y) deep. expect(x).toBeNull(), toBeDefined(), toBeTruthy(). expect(n).toBeGreaterThan(1). expect(arr).toContain(item). expect(fn).toThrow().

Async and mock

await expect(promise).resolves.toBe(val). jest.fn(), mock.mockReturnValue(1), mock.mockResolvedValue(1). vi.fn() in Vitest. describe, it, beforeEach.