I know exactly where this is from. This has been floating around the Chinese Internet for a bit. The repo is originally at https://github.com/wheatup/evil.js but has been made private since then. A few variants of this was made and uploaded to NPM.

Here's a English translation of the README.md in that specific repo.

What? The notorious 996 company wants you to hit the road?

Do you want to leave a small "gift" for your project before you go?

Let's sneak this project into yours, and your project will have, but not limited to, the following magical effects:

- When the length of the array is divisible by 7, Array.includes will always return false.

- When it's Sunday, the result of the Array.map method always loses the last element.

- There is a 2% chance that the result of Array.filter will lose the last element.

- setTimeout will always trigger one second slower than expected.

- 10% of Promise.then will not register on Sundays.

- JSON.stringify will change uppercase I to lowercase l.

- The result of Date.getTime() will always be one hour behind.

- There is a 5% chance that localStorage.getItem will return an empty string.

see: https://juejin.cn/post/7133134875426553886