> For the complete documentation index, see [llms.txt](https://xethya.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://xethya.gitbook.io/docs/utilities/assert.md).

# Assert

This is a very simple function that throws an `AssertionError` typed error with a custom message when a condition is not met.

Since it throws an error, usage of `assert` **will** halt execution of the current script.

## Usage

This is the syntax for using `assert`:

```typescript
import { assert } from '@xethya/utils';

assert(condition, 'message');
```

## Example

Let's assume you're creating a custom version of the [`Die`](/docs/components/dice.md) class, adding a `color` property, and you want to make sure the provided color is a valid hexadecimal number.

Use the `assert` function to test the parameter's value against a regular expression, and provide a readable error message when the condition isn't met.

```typescript
import { assert } from '@xethya/utils';
import { Die } from '@xethya/dice';

const hexColorPattern = /^#?[0-9a-fA-F]{3}[0-9a-fA-F]{3}?$/;

class ColoredDie extends Die {
  public readonly color: string;

  constructor(faces: number, color: string) {
    super(faces);
    assert(color.match(hexColorPattern), 'You must provide a valid hexadecimal color');
  }
}
```