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How to mock Date with Jest

Maxence Poutord on November 29, 2019

I spent too many hours trying to mock correctly the JavaScript's Date object. I tried a few things I've found on Google... without success. I final...
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clint •

Thanks for sharing this--very helpful.

The approach described here worked well for me, but then I stumbled onto another option that uses Jest's own API (I don't believe was available when this post was originally written). You can now do something like this:

import { jest } from '@jest/globals'; // Tell Jest to use a different timer implementation. You can also // configure this in your jest.config.js file. For more info see // https://jestjs.io/docs/en/configuration#timers-string). jest.useFakeTimers('modern'); jest.setSystemTime(new Date('04 Dec 1995 00:12:00 GMT').getTime()); console.log(new Date()); // "2014-01-09T00:00:00.000Z" // Back to reality... jest.useRealTimers(); console.log(new Date()); // prints the actual system time...
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Stanley Sathler • • Edited

Unfortunately jest.useFakeTimers seems to not work well with native Promises, which means you can't use it in an async call.

github.com/facebook/jest/issues/10221

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Rafael Rozon •

Thank you for this!

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Maxence Poutord •

Thanks! That's true, it was added last may with Jest 26 :)

jestjs.io/blog/2020/05/05/jest-26

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AbhishriDwivedi •

setSystemTime is giving error- setSystemTime does not exist on jest

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Avalander • • Edited

I just want to point out that both Date and RealDate reference the same object, so when you replace the function Date.now, it's also changing RealDate.now and your global.Date = RealDate at the end is not doing what you think it's doing.

What you should do if you want to keep and reassign the original now function is keep a reference to that function:

const realNow = Date.now Date.now = () => 1000 Date.now = realNow
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Angelo Cicero •

Very helpful!

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Maxence Poutord • • Edited

🙊Oh! You're completely right! Thank you so much, I will update my post!!! :)

(sorry for the late answer, I didn't saw the notification...)

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ryandowd •

Great! Just what I was looking for!

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Natalia Paz Herrera Neira •

Thanks for this process.env.TZ = 'GMT'; ❤️

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André Crimberg •

Thanks for the process.env.TZ = 'GMT' tip 🙏🏽

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Vladislav •

Great job!
Thank you a lot, it really helped.