faker.number.int() returns numbers divisible by 2^[1...21] for high maximums

[michal-kocarek]

Pre-Checks

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• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.
• I am willing to provide a PR.

Describe the bug

This investigation started by finding out that faker.number.int() % 4 is always true, regardless of the seed, regardless of how many times I query the value.

We had following code in our tests:

const { faker } = require('@faker-js/faker');

function rand(N) {
   return faker.number.int() % N;
}

// No matter how many times called, which seed is given, this always applies:
rand(4) === 0;

The issue happens only for pretty large numbers (see my investigation below). For example, if I call it with max: 10000, numbers are correctly divisible. I suspect that the issue is somewhere when MAX_SAFE_INT bounds are exceeded within number.int() calculation. There is multiplication of the random number, which (probably) loses the precision.

---

I know that this isn't how to use faker. I should have provided min and max values to int() method instead, which is what am doing right now. However, this is something that should at least deserve warning in the documentation, so people know that for numbers higher than x, precision (and randomness) is being lost.

Minimal reproduction code

const assert = require('node:assert');
const { faker } = require('@faker-js/faker');

function rand(N) {
  return faker.number.int() % N;
}

for (let i = 0; i < 100; ++i) {
  assert(rand(4) === 0);
}
console.log('All 100 random numbers are divisible by 4');

Additional Context

I was playing with following script to see how big precision is being lost. You can move EXPONENT variable up or down and will visually see that remainders start to become random.

const { faker } = require('@faker-js/faker');

const EXPONENT = 21;

faker.seed();

console.log(`faker.number.int() % (2^ EXPONENT = ${EXPONENT} )`);
for (let i = 1; i <= 100; i++) {
  const number = faker.number.int({ min: 0, max: Number.MAX_SAFE_INTEGER });
  console.log(`${i.toString().padEnd(3)} ${number.toString().padEnd(16)} remainder ${number % Math.pow(2, EXPONENT)}`);
}

2	...	10	...	21	22	23	...
	...		...				...

As an outcome, we know that unbound faker.numbner.int() always returns numbers fully divisble by 2, 4, 8, ...2^21.

Environment Info

System:
    OS: macOS 15.2
    CPU: (10) arm64 Apple M1 Pro
    Memory: 106.48 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.18.0 - ~/.nvm/versions/node/v20.18.0/bin/node
    npm: 10.8.2 - ~/.nvm/versions/node/v20.18.0/bin/npm
    pnpm: 9.7.1 - ~/Library/pnpm/pnpm
  Browsers:
    Chrome: 131.0.6778.206
    Safari: 18.2
  npmPackages:
    @faker-js/faker: ~8.4.1 => 8.4.1

Which module system do you use?

• CJS
• ESM

Used Package Manager

pnpm

[ST-DDT]

Thanks for the detailed testing.

Have you tested this with faker v9 as well?

• https://v9.fakerjs.dev/about/announcements/2024-10-26.html#use-high-precision-rng-by-default
• https://v9.fakerjs.dev/guide/upgrading.html#use-high-precision-rng-by-default

[michal-kocarek]

I have checked the number generation in v9, and it works correctly. High numbers returned by faker.number.int() behave much more as expected.

I apologize for not running the bug in latest version first.
I think this bug can be closed.

[ST-DDT]

No problem thanks for your detailed testing again.

If you need the same behavior for v8 you can create a new Faker instance and pass the 53bit randomizer.

faker/src/faker.ts

Lines 155 to 163 in 98ff809

	/**
	* The Randomizer to use.
	* Specify this only if you want to use it to achieve a specific goal,
	* such as sharing the same random generator with other instances/tools.
	*
	* @default generateMersenne32Randomizer()
	*/
	randomizer?: Randomizer;
	});

[michal-kocarek]

Thank you for your quick reply and advice.

The quickfix for v8 was to use smaller range. We replaced faker.number.int() % 4 with faker.number.int({min: 0, max: 3}).

As conceptual fix, we'll update to v9.