Function spyOn

packages/spy/src/index.ts:278–404

(
  object: T,
  key: K,
  accessor?: 'get' | 'set',
)

  ? Mock<Required<T>[M]>
  : never
export function spyOn<T extends object, K extends keyof T>(
  object: T,
  key: K,
  accessor?: 'get' | 'set',
): Mock<Procedure | Constructable> {
  assert(
    object != null,
    'The vi.spyOn() function could not find an object to spy upon. The first argument must be defined.',
  )

  assert(
    typeof object === 'object' || typeof object === 'function',
    'Vitest cannot spy on a primitive value.',
  )

  const [originalDescriptorObject, originalDescriptor] = getDescriptor(object, key) || []
  assert(
    originalDescriptor || key in object,
    `The property "${String(key)}" is not defined on the ${typeof object}.`,
  )
  let accessType: 'get' | 'set' | 'value' = accessor || 'value'
  let ssr = false

  // vite ssr support - actual function is stored inside a getter
  if (
    accessType === 'value'
    && originalDescriptor
    && originalDescriptor.value == null
    && originalDescriptor.get
  ) {
    accessType = 'get'
    ssr = true
  }

  let original: Procedure | undefined

  if (originalDescriptor) {
    original = originalDescriptor[accessType]
    // weird Proxy edge case where descriptor's value is undefined,
    // but there's still a value on the object when called
    // https://github.com/vitest-dev/vitest/issues/9439
    if (original == null && accessType === 'value') {
      original = object[key] as unknown as Procedure
    }
  }
  else if (accessType !== 'value') {
    original = () => object[key]
  }
  else {
    original = object[key] as unknown as Procedure
  }

  const originalImplementation = ssr && original ? original() : original
  const originalType = typeof originalImplementation

  assert(
    // allow only functions
    originalType === 'function'