When I'm testing out something that needs a generated environment (sprites, objects, whatever), I often use Math.random() with some ranges to avoid having to purposefully specify values. However, if something goes wrong, it's nearly impossible to reproduce the environment.

The solution to this is to use a pseudo random number generator that is seedable, which means that if you initialize it with a specific value, it will produce the same results each time (deterministic). Given that currently JavaScript doesn't have access to a PRNG, the only way do this is to implement your own seedable generator. This is actually really difficult to do, because most algorithms assume exact calculations on known integers, such as multiplying two 32-bit integers together to get a 64-bit integer (and JavaScript does not offer that sort of specificity natively). I found one by Johannes Baag√łe that uses his implementation of the Alea algorithm, and wrapped it into a dice-ready interface, so you can do things like:

var dice = new Dice(); // defaults to +new Date() as a seed, can use any number of arguments for seeding

dice.d6(); // returns 1-6
dice.d6(2); // equivalent to 2d6
dice.d100(); // d2 - d100 are valid options
dice.d49();

dice.d6(2); // returns the total as a number
dice.d6(2, true); // returns two rolls as an array