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# no-unsafe-optional-chaining

[Added in v0.4.2](https://github.com/web-infra-dev/rslint/releases/tag/v0.4.2)
## Configuration

| Preset                   | Configured Value |
| ------------------------ | ---------------- |
| ✅ js.configs.recommended | `"error"`        |


```ts title=rslint.config.ts
import { defineConfig, js } from '@rslint/core';

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'no-unsafe-optional-chaining': 'error',
    },
  },
]);
```

## Rule Details

Disallows using optional chaining in contexts where the `undefined` value is not allowed. Optional chaining (`?.`) can short-circuit to `undefined`. When the result is used in a position where `undefined` causes a TypeError or unexpected behavior, this rule reports it.

Examples of **incorrect** code for this rule:

```javascript
(obj?.foo)(); // TypeError if obj?.foo is undefined
(obj?.foo).bar; // TypeError
new (obj?.foo)(); // TypeError
const { a } = obj?.foo; // TypeError (destructuring undefined)
[...obj?.foo]; // TypeError (spreading undefined)
for (const x of obj?.foo) {
} // TypeError (iterating undefined)
'foo' in obj?.bar; // TypeError
foo instanceof obj?.bar; // TypeError
class Foo extends obj?.bar {} // TypeError
```

Examples of **correct** code for this rule:

```javascript
obj?.foo; // standalone is fine
obj?.foo(); // optional call is fine
(obj?.foo ?? bar)(); // fallback via ??
(obj?.foo || bar).baz; // fallback via ||
obj?.foo?.bar; // chaining is fine
```

## Options

### `disallowArithmeticOperators`

When set to `true`, also reports arithmetic operations on optional chaining results, which can produce `NaN`. Default is `false`.

Examples of **incorrect** code with `{ "disallowArithmeticOperators": true }`:

```javascript
obj?.foo + bar; // may be NaN
+obj?.foo; // may be NaN
-obj?.foo; // may be NaN
obj?.foo * bar; // may be NaN
```

## Original Documentation

- [ESLint: no-unsafe-optional-chaining](https://eslint.org/docs/latest/rules/no-unsafe-optional-chaining)
- [Source code](https://github.com/eslint/eslint/blob/v10.8.1/lib/rules/no-unsafe-optional-chaining.js)