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Jest / Vitest assertions

Updated 2026-02-15

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

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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.