For duplicate elimination, it's important to have deterministic execution of Javascript, for duplicate elimination. Getting several identical pages (with different URLs) in your search results is a really bad user experience.
As of when I left Google in 2010, the JavaScript random number generator always returned 0.5 (and some SEO figured it out and blogged about it, no secrets here). However, I was trying to convince my manager to let me instead seed a random number generator with an HMAC of all of the currently loaded HTML and JavaScript (to make it deterministic but hard to display something good 1 in a million times but 100% of the time to Google's indexing system).
As of when I left Google in 2010, the JavaScript random number generator always returned 0.5 (and some SEO figured it out and blogged about it, no secrets here). However, I was trying to convince my manager to let me instead seed a random number generator with an HMAC of all of the currently loaded HTML and JavaScript (to make it deterministic but hard to display something good 1 in a million times but 100% of the time to Google's indexing system).