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Julio Lugo
Julio Lugo

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Test dates-related and time features using date-fns and Jest

So, one of the issues I was wondering when I started working with Jest was: "How can I test dates and times?" and I felt curious so I started Googling it. At the very beginning, my findings were zero to none and I felt quite down with it, so I decided to experiment on my own... Total failure again.

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However, after hours of suffering and reading a bunch of random blogs across the web, I found a simple and straightforward way to achieve this. It starts simply with configuring a fixed TZ for all tests.

// In your jest.config.js process.env.TZ = 'GMT' module.exports = { // ... }
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Then, in your .spec file, you need to alter your global Date object and define a jest function for the built-in now function, as follows:

const RealNow = Date.now beforeAll(() => { global.Date.now = jest.fn(() => new Date('2019-04-07T10:20:30Z').getTime()) }) afterAll(() => { global.Date.now = RealNow })
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Once you have it up and running, you can create a very simple test to make sure everything is alright

it ('should show time', () => { expect(getHumanizeDate('2000/01/03')).toEqual('today') ... })
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And the code

import { isToday } from 'date-fns' export const getHumanizeDate = (date: string): string => { const dateToCheck = new Date(Date.parse(date)) return isToday(dateToCheck)
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Sources:

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