Generate Repeatable Random Numbers (in JS)

Quite often when developing a game I need “random” numbers, that I can reproduce or that I can steer. Why? – For example when setting up a level or when resetting a level you might want to have it look random, but still the same every time you load it. – So how do we do that? There are tons of ways of achieving a so called “seeded random generated number” – if you want any details on the algorithms you can for example check out this link. But basically they all work the same: You initialize the algorithm with a “seed” (a number, that you choose to steer the algorithm). And every seed will return its own unique set of “random” numbers, so e.g.: Every time I seed the Number 6, I get the same set of generated “random” numbers. Every time I seed the Number 10, I get the same set of “random” number, that is different from the set ‘6’ and so on…
Because of this fact: seeded random numbers are predictable, which is not bad, but just don’t use it for encryption purposes ect… ;-)

The algorithm

So the algorithm I’m using is the following, it’s really very simple:
(this is nothing new and also nothing I invented, but I think this might help quite a few people)

You may ask: Why ‘(seed * 9301 + 49297) % 233280‘ ?!
The answer is both simple&complicated: The combination of 9301, 49297 and 233280 provide a very even distributed set of “random” numbers. Please don’t ask WHY – that’s the complicated part, some very smart people figured out those numbers quite some time ago, and I also cannot tell you how they did it. But as allways: Google is your friend ;-)


I set up a small benchmark on jsfiddle, to test the performance of that algorithm and also how the distribution of the generated sets are: Just hit then Run-button and see the results, for me Math.random() is usually 3-4x faster than Math.seededRandom(), which is for 100.000 numbers still within the range of 10-15ms and therefor pretty fast. Also the distribution of the numbers is very equal as you can see.

See it in ‘action’

I quickly implemented this in my Runner Game Prototype, so no matter how often you reload the page, the level is allways going to look the same and all I did was replacing the “Math.random()” with “Math.seededRandom()” and of course I added the algorithmto the utilities-file.

If you want to try out other seeds, just go to: and change the “seed” in the query string.

  • ngryman

    Usefull thanks :)

  • v

    You have impossibly long jumps in that game :(

    • Spoygg

      there is double jump though

  • Francis Norton

    Awesome – exactly what I needed, in working code plus a great explanation. Thanks!

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  • aterra103

    Exactly what I was looking for, thanks

  • Roger Heathcote

    Thanks for that, just what I needed for testing some code that uses random numbers!