@typescript-eslint/eslint-plugin/docs/rules/non-nullable-type-assertion-style.mdx

TypeScript plugin for ESLint

---
description: 'Enforce non-null assertions over explicit type casts.'
---

import Tabs from '@theme/Tabs';
import TabItem from '@theme/TabItem';

> 🛑 This file is source code, not the primary documentation location! 🛑
>
> See **https://typescript-eslint.io/rules/non-nullable-type-assertion-style** for documentation.

There are two common ways to assert to TypeScript that a value is its type without `null` or `undefined`:

- `!`: Non-null assertion
- `as`: Traditional type assertion with a coincidentally equivalent type

`!` non-null assertions are generally preferred for requiring less code and being harder to fall out of sync as types change.
This rule reports when an `as` cast is doing the same job as a `!` would, and suggests fixing the code to be an `!`.

## Examples

<Tabs>
<TabItem value="❌ Incorrect">

```ts
const maybe: string | undefined = Math.random() > 0.5 ? '' : undefined;

const definitely = maybe as string;
const alsoDefinitely = <string>maybe;
```

</TabItem>
<TabItem value="✅ Correct">

```ts
const maybe: string | undefined = Math.random() > 0.5 ? '' : undefined;

const definitely = maybe!;
const alsoDefinitely = maybe!;
```

</TabItem>
</Tabs>

## When Not To Use It

If you don't mind having unnecessarily verbose type assertions, you can avoid this rule.