JavaScript - own random number generator (custom implementation)

In this article, we're going to have a look at how to write own random numbers generator that is based on LCG (Linear congruential generator) algorithm in JavaScript.

1. Introduction

In JavaScript, we can implement a custom random number generator by using LCG (Linear congruential generator) algorithm. LCG is one of the oldest and best-known pseudorandom number generator algorithm. We can adjust this implementation to work on int or long.

Linear congruential generator is defined by the recurrence formula:

Where:

• Xn+1 - new seed,
• a - multiplier,
• Xn - current seed (comes from previous calculations),
• c - incrementer,
• m - modulus (max integer value).

2. Implementation

Note: every time randomInt() method is called, it is necessary to update seed value.

// ONLINE-RUNNER:browser;

function CustomRandom(seed) {
    var self = this;
    if (seed == null) {
        seed = Date.now();
    }
    var A = 1103515245; // multiplier
    var C = 12345;      // incrementer
    var M = 2147483647; // modulus == max integer value
    self.nextInt = function() {
        seed = (seed * A + C) % M;
        return seed;
    };
  	self.nextDouble = function() {
        var value = self.nextInt();
        return value / M;
    };
}


// Usage example 1:

var random = new CustomRandom(3819201); // 3819201 is example initial seed

console.log(random.nextInt());  // 348328093
console.log(random.nextInt());  // 121065958
console.log(random.nextInt());  // 790325445

// Usage example 2 (values should be different always):

var random = new CustomRandom();

console.log(random.nextInt());  // 922892575
console.log(random.nextInt());  // 1117277734
console.log(random.nextInt());  // 2084933141

console.log(random.nextDouble());  // 0.6316955399847103
console.log(random.nextDouble());  // 0.5716405932659472
console.log(random.nextDouble());  // 0.3298228519641901