# required

## Effect

Restrict the detected value is not `undefined | null`.

## Arguments

None

## Example

```javascript
const schema1 = racoon.number();
schema1.validate(undefined); // pass, return undefined
schema1.validate(null); // pass, return null

const schema2 = racoon.number().required();
schema2.validate(undefined); // fail
schema2.validate(null); // fail
```


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