how-to-mock-third-party-es6-exports-in-vitest.ts

/**
 * Check out the blog post, "How to mock a third party ES6 export in Vitest", at:
 * https://dev.to/erikpuk/how-to-mock-a-third-party-es6-export-in-vitest-38ff
 */

// Import the module:
import * as Firestore from "firebase/firestore"
import { test, vi } from "vitest"

type FakeFirestore = { onSnapshot(this: void): void }

// Mock the import:
vi.mock("firebase/firestore", async (getModule) => {
  const original: FakeFirestore = await getModule()

  return {
    ...original,
    onSnapshot: vi.fn().mockImplementation(original.onSnapshot),
  }
})

test("calls onSnapshot", () => {
  // Create your spy:
  const onSnapshot = vi.spyOn(Firestore, "onSnapshot")

  /* trigger the onSnapshot call here in my code */

  // Place expectations on the spy:
  expect(onSnapshot).toHaveBeenCalled()
})