> For AI agents: the complete documentation index is available at https://rslint.rs/llms.txt, the full documentation bundle is available at https://rslint.rs/llms-full.txt.

# prefer-set-size

[Added in v0.9.3](https://github.com/web-infra-dev/rslint/releases/tag/v0.9.3)
## Configuration

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


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

export default defineConfig([
  unicornPlugin.configs.recommended,
  {
    rules: {
      'unicorn/prefer-set-size': 'error',
    },
  },
]);
```

Prefer using `Set#size` instead of `Array#length`.

Converting a `Set` into an array solely to read its length performs unnecessary
work. Use its `size` property directly.

## Examples

```js
// Incorrect
const uniqueCount = [...new Set(userIds)].length;

// Correct
const uniqueCount = new Set(userIds).size;
```

```js
// Incorrect
const count = Array.from(items).length;

// Correct
const count = items.size;
```

## Original Documentation

- [eslint-plugin-unicorn: prefer-set-size](https://github.com/sindresorhus/eslint-plugin-unicorn/blob/v74.0.0/docs/rules/prefer-set-size.md)
- [Source code](https://github.com/sindresorhus/eslint-plugin-unicorn/blob/v74.0.0/rules/prefer-set-size.js)