如何在执行导入之前在玩笑中模拟全局变量？

Question

The value of epoch is set early, and the function has a closure on it as I dont want to regenerate the value of epoch.

/* index.js */
const epoch = Date.now()

function appVersion() {
  console.log(epoch)
}

export { appVersion }

The issue I encounter is that no matter how I mock the global Date object in my test file, this one would return the value not mocked since the import is executing first.

How can I mock Date.now() in my test file?

The alternative I used for now is to have Date.now() in a separate file and jest.mock the file imported

/* epoch.js */
const epoch = Date.now()

export = { epoch }

/* index.test.js */
import { appVersion } from './app-version'

jest.mock('./epoch', () => ({
  epoch: '11111111'
})

Answer 1

A common pattern for dealing with dates and other non-deterministic values ( Math.random() for example) is to pass them in as an argument, and give them a default value.

const defaultEpoch = Date.now();

function appVersion(epoch = defaultEpoch) {
  console.log(epoch)
}

export { appVersion }

This way, the code works as expected, but you now have the ability to pass in a specific value when you're testing, in order to make it deterministic:

import { appVersion } from './app-version'

it('uses epoch value for appVersion', () => {
  const mockEpoch = '11111111';
  const version = appVersion(mockEpoch);
  expect(version).toBe('11111111');
};