# bun/fast-check-no-unseeded-random

> Disallow `Math.random()` and `fc.sample()` inside a fast-check property

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.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

Off by default. To turn it on:

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

{/* End of the generated section. */}

## Examples

Reported:

{/* bad */}
```ts
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;
  }),
);
```

{/* bad */}
```ts
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:

{/* good */}
```ts
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:

{/* good */}
```ts
import fc from "fast-check";

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