Javascript does not allow you to set the seed for the default PRNG. So for our purposes it's essentially non-deterministic. You need to use a custom library if you want to be able to specify your seed.
Continuing the nitpicking: "JavaScript" isn't actually what prevents setting the seed, you're talking about a common implementation detail of specific JavaScript runtimes (Mozilla’s documentation says their seed can't be changed, for instance). A runtime developer could, as Googlebot apparently does, seed the prng with a constant value. You could also use some user-provided constant as the seed. All three are compliant with the letter of the spec.
Bottom line: the PRNG is, as all PRNGs are, deterministic. The user-facing math.random API using the underlying PRNG may or may not be. Those are distinct things.
From the spec's description of math.random with regard to random or pseudo-random:
"...using an implementation‑
dependent algorithm or strategy."
Technically true, but pointless pedantry when I'm sure that most people are on the the same page wrt pseudorandomness. Typically what I'd expect of /r/programming.