For tutorial: Testing JavaScript Code with Timeouts and Promises

handleFetchedMessage.spec.js

import { jest } from '@jest/globals';
import * as FetchUtils from "../fetchMessage.js";
import { handleFetchedMessage } from "../handleFetchedMessage.js";

describe("handleFetchedMessage", () => {
  beforeEach(() => {
    // 1
    jest.useFakeTimers()
  })

  afterEach(() => {
    // 2
    jest.runOnlyPendingTimers()

    // 3
    jest.useRealTimers()

    jest.resetAllMocks()
  })

  it("should handle fetched message correctly", async () => {
    const mockCallback = jest.fn()

    jest.spyOn(FetchUtils, "fetchMessage").mockResolvedValue("Mock message")

    handleFetchedMessage(mockCallback)

    // 4
    jest.runAllTimers()

    // 5
    await Promise.resolve()

    expect(mockCallback).toHaveBeenCalledTimes(1)
    expect(mockCallback).toHaveBeenCalledWith("Mock message")
  })
})