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 (32 bits) or long (64 bits) types - check parameters section. The algorithm works on and returns positive values.

The linear congruential generator is defined by the recurrence formula:

Where:

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

AND used m, a, and c parameters should keep conditions:

• 0 < m
• 0 < a < m
• 0 <= c < m
• 0 <= X0 < m

2. Implementation

In this section, you can find custom implementation that uses BigInt type to prevent against integer overflow. During double values calculations we use Number type, ignoring least significant bits in integers that do not have impact during that calculations.

// ONLINE-RUNNER:browser;

function CustomRandom(seed) {
    if (seed < 0) {
        throw new Error('Seed value must be positive.');
    }
    const M = 2147483648n;  // modulus (max possible integer value)
    const A = 1103515245n;  // multiplier
    const C = 12345n;       // incrementer
    let R = BigInt(seed ?? Date.now()) % M;
    this.nextInt = () => {
        R = (R * A + C) % M;
        return R;
    };
  	this.nextDouble = () => {
        const value = this.nextInt();
        return Number(value) / Number(M);
    };
}



// Usage example 1:
{
    const 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 always be different):
{
    const 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
}

Note: BigInt type was introduced around 2018-2020 in the major web browsers.

3. LCG algorithm parameters

In this section, we can find parameters that are in common use by LCG.

From Wikipedia:

Source: https://en.wikipedia.org/wiki/Linear_congruential_generator#Parameters_in_common_use

References

1. Linear congruential generator (LCG) - Wikipedia

2. Linear congruential generator: Parameters in common use - Wikipedia