# custom

## Effect

Declare a custom restrict by a custom `callback` function.

## Arguments

* `callback` ***(Function)*** - The custom function for restricting. *`callback` has a parameter: the detected value.* If validate failed, the callback should throw an error, otherwise, it means validation passed.
* `[ctx]` ***(\*)*** - The execution context of `callback`.

> **NOTE**
>
> Since 1.3.0, `callback` can return a non-empty string to respect that validating failed, the string means the failure reason. Otherwise, it means validating passed if you return other values.

## Example

```javascript
const schema = racoon.object().custom((val) => {
  if (Object.keys(val) === 1) {
    return true;
  }
  throw new Error('The detected object must have only one key');
});
schema.validate({ a: 1 }); // pass
schema.validate({ a: 1, b: 2 }); // fail
```