Why Can't You Seed JavaScript's Math.random(), and What's a Better Alternative?

The Inability to Seed Math.random() and a Seedable Alternative

The JavaScript Math.random() function, as defined by the ECMAScript specification, provides a means to produce pseudo-random numbers but lacks the capability to be seeded. This means the generated sequence of numbers cannot be controlled or repeated.

Why No Seeding in Math.random()

The ECMAScript specification intentionally omits seeding to avoid uniformity among browsers. This approach provides flexibility for browser vendors to employ different algorithms, optimizing performance or randomness as they see fit.

A Seedable Alternative

In lieu of seeding Math.random(), external functions can be utilized as seedable PRNGs (Pseudorandom Number Generators). These functions mimic the behavior of Math.random() closely, offering the ability to initialize with a specific seed state. This allows for consistent and reproducible random sequences.

Implementing Seedable PRNG

To effectively generate seedable PRNG, the initial seed state is crucial. Consider employing hash functions to convert short strings into seeds or pad seed values with dummy data to enhance entropy. For optimal results, discard the initial few numbers (~15) generated by the PRNG to avoid correlation issues.

Example Seedable PRNGs

Among the available seedable PRNG algorithms, sfc32 stands out for its speed and compatibility with JavaScript. Its core uses 32-bit operations, ensuring performance without the need for complex shims.

With a seeded PRNG, developers can generate repeatable random sequences for various applications such as game development or simulations, ensuring consistent and controlled randomness.

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