According to this post on the official V8 Javascript blog, the pseudo-random number generator (PRNG ... For the last six years, the algorithm that’s been used has been pretty horrible.

only the difference of a few milliseconds between keystrokes is enough to seed a random number generation routine with a different starting number each time. Once seeded, an algorithm computes ...

This returns the required number, and you go happily on your way. A shift register configured as a pseudo-random number generator ... and if a PRNG algorithm can be started with a seed derived ...

Modern applications rely on surprisingly complex mathematical algorithms to produce the randomness we take for granted.