This is a primality testing algorithm written in javascript and geared towards being as fast as possible. It is an adaptation of https://github.com/latonv/MillerRabinPrimality optimized for speed and determinism for "small" inputs.
isPrime.js contains the function primalityTest(n) which returns a boolean value specifying whether n is a probable prime or not, where n can be either a number or a BigInt or a string. Look at the comments in the code for the detailed description of every function and the optional arguments that can be specified.
We employ the Miller-Rabin algorithm, and we utilize the Montgomery modular multiplication method for large inputs above 10³⁰ by default. For inputs below 2⁶⁴, our algorithm was written to be deterministic and always test the optimal bases (see an explanation here). For inputs larger than 2⁶⁴, the algorithm is probabilistic and the number of bases tested is adjusted dynamically by the method getAdaptiveNumRounds(inputBits), finding a good tradeoff between speed and reliability.
This code was adapted by Andreas Tsevas and Naviary for a private project, but feel free to use it as part of your own project if you need a browser to calculate primes in natively written javascript.