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# prefer-const

[Added in v0.3.3](https://github.com/web-infra-dev/rslint/releases/tag/v0.3.3)
## Configuration

| Preset                              | Configured Value |
| ----------------------------------- | ---------------- |
| ✅ ts.configs.recommended            | `"error"`        |
| ✅ ts.configs.recommendedTypeChecked | `"error"`        |
| ✅ ts.configs.strict                 | `"error"`        |
| ✅ ts.configs.strictTypeChecked      | `"error"`        |
| ✅ ts.configs.stylistic              | `"error"`        |
| ✅ ts.configs.stylisticTypeChecked   | `"error"`        |


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

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'prefer-const': 'error',
    },
  },
]);
```

## Rule Details

Requires `const` declarations for variables that are never reassigned after declared. If a variable is never reassigned, using the `const` declaration is better because it makes the intent clear that the value is not intended to be changed.

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

```javascript
let x = 1;
let obj = { key: 0 };
for (let x in obj) {
  console.log(x);
}
for (let x of [1, 2, 3]) {
  console.log(x);
}
```

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

```javascript
const x = 1;
const obj = { key: 0 };
let y = 1;
y = 2;
let z;
z = 1;
for (const x in obj) {
  console.log(x);
}
for (const x of [1, 2, 3]) {
  console.log(x);
}
```

A global named by an `/* exported name */` block comment is shared with the
other scripts loaded alongside this one, any of which may reassign it, so the
rule leaves such a declaration alone:

```javascript
/* exported sharedValue */
let sharedValue = 1;
```

## Original Documentation

- [ESLint: prefer-const](https://eslint.org/docs/latest/rules/prefer-const)
- [Source code](https://github.com/eslint/eslint/blob/v10.8.1/lib/rules/prefer-const.js)