# Jest Cheatsheet

## `spyOn` modules

// myModule.ts

export function foo () {
  return true
}

// anotherModule.spec.ts

import * as myModule from './myModule'

const spyOnMyModule = jest.spyOn(myModule, 'foo')

spyOnMyModule.mockReturnValue(true)

## 模擬 HTMLElement 部分屬性

const table = document.createElement('div');
const mockTableSetScrollTop = jest.fn()

Object.defineProperty(table, 'scrollTop', {
  get: () => 500,
  set: mockTableSetScrollTop,
});

## 檢查函式呼叫帶的參數是否部分匹配

import * as myModule from '...'

const spyOnMyModule = jest.spyOn(myModule, 'foo')

// Method 1:
expect(
  spyOnMyModule.mock.lastCall[0]
).toMatchObject({ ... });

// Method 2:
expect(
  spyOnMyModule.mock.call[0][0]
).toMatchObject({ ... });

// Method 3:
expect(spyOnMyModule).toBeCalledWith(
  expect.objectContaining({
    ...
  })
);

## 留意多個 `describe()` 之間的執行順序

describe('describe A', () => {
  console.log('1')
  beforeAll(() => console.log('3'))
})

describe('describe B', () => {
  console.log('2')
  beforeAll(() => console.log('4'))
})

執行順序：`1 → 2 → 3 → 4`

在 `describe` 區塊直接呼叫 `jest.spyOn()`，可能會導致後面的 describe B 把前面 describe A 的給覆蓋掉。

## 測試元素的某個方法有無被呼叫

it('should scroll to end', async () => {
  const mockScrollTo = jest.fn();
  window.HTMLElement.prototype.scrollTo = mockScrollTo;
  const wrapper = mount(App);
  wrapper.vm.scrollToEnd();

  expect(mockScrollTo).toBeCalled();
});

## 計算覆蓋率

jest --coverage 

jest --coverage --collectCoverageFrom=<file-path>

## 檔案變動時自動重跑測試

jest --watch 

jest --watchAll