TypeError: expect.createSpy is not a function

在使用Expect進行測試過程中，遇到TypeError: expect.createSpy is not a function 該問題，那是因爲：

This package has been donated to Jest. This means that all future development of expect v21+ will take place at facebook/jest.

因此，如果你使用的是21+版本，應該使用fest.fn代替expect.createSpy
具體操作如下：

import jest from 'jest-mock';
const spy=fest.fn();
spy();

參考：https://stackoverflow.com/questions/46752389/typeerror-expect-createspy-is-not-a-function
https://stackoverflow.com/questions/46086970/getting-typeerror-jest-fn-is-not-a-function