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vite
chore: keep length static (#8782)
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docs/api/mock.md
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docs/api/mock.md
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You should use mock assertions (e.g., [`toHaveBeenCalled`](/api/expect#tohavebeencalled)) on [`expect`](/api/expect) to assert mock results. This API reference describes available properties and methods to manipulate mock behavior.
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::: warning IMPORTANT
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Vitest spies inherit implementation's `length` property. This means that `length` can be different from the original implementation:
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Vitest spies inherit implementation's [`length`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/length) property when initialized, but it doesn't override it if the implementation was changed later:
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```ts
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::: code-group
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```ts [vi.fn]
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const fn = vi.fn((arg1) => {})
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fn.length // == 1
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fn.mockImplementation(() => {})
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fn.length // == 1
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```
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```ts [vi.spyOn]
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const example = {
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fn(arg1, arg2) {
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// ...
···
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fn.length // == 2
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fn.mockImplementation(() => {})
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fn.length // == 0
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fn.length // == 2
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```
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:::
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packages/spy/src/index.ts
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packages/spy/src/index.ts
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state,
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...options,
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})
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const mockLength = (mockImplementation || originalImplementation)?.length ?? 0
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Object.defineProperty(mock, 'length', {
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writable: true,
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enumerable: false,
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value: mockLength,
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configurable: true,
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})
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// inherit the default name so it appears in snapshots and logs
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// this is used by `vi.spyOn()` for better debugging.
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// when `vi.fn()` is called, we just use the default string
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if (original) {
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copyOriginalStaticProperties(namedObject[name], original)
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}
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let overrideLength: number | undefined
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Object.defineProperty(namedObject[name], 'length', {
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configurable: true,
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get: () => {
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if (overrideLength != null) {
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return overrideLength
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}
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const implementation = config.onceMockImplementations[0]
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|| config.mockImplementation
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|| prototypeConfig?.onceMockImplementations[0]
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|| prototypeConfig?.mockImplementation
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|| original
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return implementation?.length ?? 0
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},
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set: (length: number) => {
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overrideLength = length
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},
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})
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return namedObject[name]
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}
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test/core/test/mocking/vi-fn.test.ts
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test/core/test/mocking/vi-fn.test.ts
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}).toThrowError()
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})
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test('vi.fn() has correct length', () => {
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const fn0 = vi.fn(() => {})
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expect(fn0.length).toBe(0)
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describe('fn.length is consistent', () => {
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test('vi.fn() has correct length', () => {
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const fn0 = vi.fn(() => {})
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expect(fn0.length).toBe(0)
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const fnArgs = vi.fn((..._args) => {})
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expect(fnArgs.length).toBe(0)
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const fnArgs = vi.fn((..._args) => {})
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expect(fnArgs.length).toBe(0)
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const fn1 = vi.fn((_arg1) => {})
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expect(fn1.length).toBe(1)
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const fn1 = vi.fn((_arg1) => {})
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expect(fn1.length).toBe(1)
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const fn2 = vi.fn((_arg1, _arg2) => {})
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expect(fn2.length).toBe(2)
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const fn2 = vi.fn((_arg1, _arg2) => {})
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expect(fn2.length).toBe(2)
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const fn3 = vi.fn((_arg1, _arg2, _arg3) => {})
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expect(fn3.length).toBe(3)
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const fn3 = vi.fn((_arg1, _arg2, _arg3) => {})
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expect(fn3.length).toBe(3)
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})
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test('vi.fn() always returns length of the initial implementation', () => {
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const fn = vi.fn<(...args: any[]) => void>((_arg) => {})
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expect(fn.length).toBe(1)
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fn.mockImplementationOnce(() => {})
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expect(fn.length).toBe(1)
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fn(1)
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expect(fn.length).toBe(1)
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fn.mockImplementation((_arg1, _arg2) => {})
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expect(fn.length).toBe(1)
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fn(1)
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expect(fn.length).toBe(1)
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fn.withImplementation((_arg1, _arg2, _arg3) => {}, () => {
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expect(fn.length).toBe(1)
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})
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})
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})
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test('vi.fn() has overridable length', () => {