The blog of dlaa.me

JavaScript's Math.random() function has a well-deserved reputation for not generating truly random numbers. (Gasp!) Modern browsers offer a solution with the crypto.getRandomValues() function that new code should be using instead. However, most legacy scripts haven't been - and won't be - updated for the new hotness.

I wanted to improve the behavior of legacy code and looked around for a polyfill of Math.random() that leveraged crypto.getRandomValues() to generate output, but didn't find one. It seemed straightforward to implement, so I created math-random-polyfill.js with the tagline: A browser-based polyfill for JavaScript's Math.random() that tries to make it more random.

You can learn more about math-random-polyfill.js on its GitHub project page which includes the following:

The MDN documentation for Math.random() explicitly warns that return values should not be used for cryptographic purposes. Failing to heed that advice can lead to problems, such as those documented in the article TIFU by using Math.random(). However, there are scenarios - especially involving legacy code - that don't lend themselves to easily replacing Math.random() with crypto.getRandomValues(). For those scenarios, math-random-polyfill.js attempts to provide a more random implementation of Math.random() to mitigate some of its disadvantages.

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math-random-polyfill.js works by intercepting calls to Math.random() and returning the same 0 <= value < 1 based on random data provided by crypto.getRandomValues(). Values returned by Math.random() should be completely unpredictable and evenly distributed - both of which are true of the random bits returned by crypto.getRandomValues(). The polyfill maps those values into floating point numbers by using the random bits to create integers distributed evenly across the range 0 <= value < Number.MAX_SAFE_INTEGER then dividing by Number.MAX_SAFE_INTEGER + 1. This maintains the greatest amount of randomness and precision during the transfer from the integer domain to the floating point domain.

I've included a set of unit tests meant to detect the kinds of mistakes that would compromise the usefulness of math-random-polyfill.js. The test suite passes on the five (most popular) browsers I tried, which leads me to be cautiously optimistic about the validity and viability of this approach. :)