# isUndefined

Evaluates whether the given value is **undefined**

```javascript
import isUndefined from 'deep-waters/isUndefined';

isUndefined(undefined); // → true
isUndefined(null); // → false
isUndefined(false); // → false
```

*isUndefined*, like any other type validator, is mostly used in [composed](https://antonioru.gitbook.io/deep-waters/type-validators/broken-reference) validators:

```javascript
import isString from 'deep-waters/isString';
import matchesPattern from 'deep-waters/matchesPattern';

const isEmail = compose(
  isString,
  matchesPattern(/^\w+([.-]?\w+)*@\w+([.-]?\w+)*(\.\w{2,3})+$/),
);
```

{% hint style="success" %}
**isUndefined** is built on top of [ofClass](https://antonioru.gitbook.io/deep-waters/validators/ofclass) validator and rely on `Object.prototype.toString` to check on the received value type in order to avoid [typeof weird behaviours ](https://stackoverflow.com/questions/18808226/why-is-typeof-null-object)and encouraging a **runtime strict type validation**.
{% endhint %}