It's easy to see some major problems, but take a look at this: https://random.isthe.link/?code=let+x+%3D+Date.now%28%29+%26...

The random number generator here is used quite often in projects and it looks pretty random. But if you look at the code you can see that it has four numbers that store state and its output can be entirely predicted from those four numbers that are consecutive outputs of the RNG.