bun/fast-check-no-unseeded-random

fast-check draws every input of a property from one seed. When a property fails, it prints the seed so you can run the same inputs again, and it shrinks the inputs to the smallest ones that still fail.

A value from Math.random() or fc.sample() inside the function you pass to fc.property() or fc.asyncProperty() doesn't come from that seed. It's different on every run, so the failure can't be replayed or shrunk. Make the value a parameter of the property instead.

Configure

Off by default. To turn it on:

[lint.rules]
"bun/fast-check-no-unseeded-random" = "error" # or "warn"

Examples

Reported:

import fc from "fast-check";
import { pad } from "./text";

fc.assert(
  fc.property(fc.string(), text => {
    const width = Math.floor(Math.random() * 80);
    return pad(text, width).length >= width;
  }),
);

import { assert, integer, property, sample, string } from "fast-check";
import { pad } from "./text";

assert(
  property(string(), text => {
    const [width] = sample(integer({ min: 0, max: 80 }), 1);
    return pad(text, width).length >= width;
  }),
);

Not reported:

import fc from "fast-check";
import { pad } from "./text";

fc.assert(
  fc.property(fc.string(), fc.integer({ min: 0, max: 80 }), (text, width) => {
    return pad(text, width).length >= width;
  }),
);

fc.sample() outside a property is fine:

import fc from "fast-check";

console.log(fc.sample(fc.emailAddress(), 5));