This answer mentions the xoshiro128** algorithm and provides a JavaScript implementation.
Question: Is it possible to derive the seed if you know the first random number produced by this generator?
>Solution :
No, it’s not possible to derive a seed given a single value. There is also no guarantee that you can derive a seed given any number of consecutive values. Even if you pre computed every possible seed value in advance its extremely unlikely that you will find only one sequence thats starts with whatever that first value is.