如何檢查一個類方法是否只用 JEST 調用過一次

Question

問題：

我應該如何檢查Service.init()方法是否只被調用過一次？

錯誤

expect(received).toHaveBeenCalledTimes(expected)

    Matcher error: received value must be a mock or spy function

    Received has type:  object
    Received has value: {"constructor": [Function Statistics], "init": [Function init]}

      13 |     const Module3 = Service
      14 |
    > 15 |     expect(Service).toHaveBeenCalledTimes(1)
         |                     ^
      16 |   })
      17 |

代碼

index.spec.js：

  it('should initialized a class instance only once', async () => {
    const Service = require('./index')
    jest.mock('./index')

    const Module1 = Service
    const Module2 = Service
    const Module3 = Service

    expect(Service).toHaveBeenCalledTimes(1)
  })

基本上是一個類代碼：

class Statistics {
  constructor () {
    console.log('📊 Constructor')
    this.init()
  }

  async init () {
    // Wait 500ms
    console.log('📊 Initializing')
    await new Promise(resolve => setTimeout(resolve('ok'), 500))
    console.log('📊 Initialization complete')
  }
}

module.exports = new Statistics()

Answer 1

在檢查之前您沒有調用該函數。 文檔中的一個示例 - 使用.toHaveBeenCalledTimes確保模擬函數被調用的確切次數。

例如，假設您有一個drinkEach(drink, Array) 函數，該函數接受一個drink 函數並將其應用於傳遞的飲料數組。 您可能想要檢查飲料函數被調用的確切次數。 你可以用這個測試套件做到這一點：

test('drinkEach drinks each drink', () => {
  const drink = jest.fn();
  drinkEach(drink, ['lemon', 'octopus']); // calling the function and checking it
  expect(drink).toHaveBeenCalledTimes(2);
});

在您的測試規范中，創建實例足以調用 init：

  it('should initialized a class instance only once', async () => {
    const Service = require('./index');
    jest.mock('./index')

    const Module1 = Service
    const Module2 = Service
    const Module3 = Service
    const mockServiceInstance = Service.init();
    expect(Service).toHaveBeenCalledTimes(1)
  })

Answer 2

就我而言，我是在調用一個方法后監視它。

這不起作用：

it('should ...', () => {
  instance = new MyClass();
  expect(jest.spyOn(instance, 'methodname')).toBeCalled();
});

但這確實：

it('should ...', () => {
  const f = jest.spyOn(instance, 'methodname');
  instance = new MyClass();
  expect(f).toBeCalled();
});