seedrand.js is a micro library which allows you to seed Math.random(). It’s mainly written for testing functions that require random numbers.

The library can be included server- and/or client-side. It moves the original Math.random to Math._random, and use a custom pseudo-random number generator (PRNG) to replace it.

<script src="/path/to/seedrand.min.js"></script>
<script>
var mySeed = +new Date(),
    a, b;

Math.seed( mySeed );
a = Math.random();

Math.seed( mySeed );
b = Math.random();

console.log( a === b ); // true
</script>

require( 'seedrand' );

var mySeed = +new Date(),
    a, b;

Math.seed( mySeed );
a = Math.random();

Math.seed( mySeed );
b = Math.random();

console.log( a === b ); // true

On the client-side, include the seedrand.min.js file in your page. On the server-side, install the seedrand npm package:

[sudo] npm -g install seedrand

The code of the PRNG is based on Adam Hyland’s implementation of “Webkit2’s crazy invertible mapping generator”.