# isDefined

> Type-narrowing predicate that filters out `null` and `undefined` — perfect as the callback for `Array.filter` when you want a strictly-typed non-nullish list.

Source: https://ngwr.dev/reference/utils/is-defined  
Kind: Util

## Usage

```angular-ts
import { isDefined } from 'ngwr/utils';

const items: (string | null | undefined)[] = ['a', null, 'b', undefined];
const present = items.filter(isDefined);   // string[] — narrowed
```

## Why ngwr provides this

TypeScript doesn't narrow through inline arrow predicates, so `arr.filter(x => x !== null && x !== undefined)` returns `(T | null | undefined)[]` — still nullable. A named function with a type-predicate signature narrows the array element. It's tiny but actually pays off in code that walks collections of optionals.

```angular-ts
// Native — TS doesn't narrow through an inline arrow predicate.
const items: (string | null | undefined)[] = ['a', null, 'b'];
const a = items.filter(x => x !== null && x !== undefined);
//    ^? (string | null | undefined)[]      ← still nullable!

// ngwr — type predicate signature narrows the result.
const b = items.filter(isDefined);
//    ^? string[]                          ← clean
```

## API

| Name | Description | Type | Default |
| --- | --- | --- | --- |
| `isDefined(v)` | Type-narrowing guard that excludes `null` and `undefined`. Composes cleanly with `Array.filter`. | `<T>(v: T \| null \| undefined) => v is T` | `—` |