# prefer-set-size

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

💼 This rule is enabled in the following [configs](https://github.com/sindresorhus/eslint-plugin-unicorn#recommended-config): ✅ `recommended`, ☑️ `unopinionated`.

🔧 This rule is automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/latest/user-guide/command-line-interface#--fix).

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[`Set#size`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/size) is a direct way to get the number of elements in a `Set`. Converting a `Set` to an array just to access its `.length` is inefficient and defeats the purpose of using a Set. The `size` property is O(1) and doesn't require any conversion.

## Examples

```js
// ❌
function isUnique(array) {
	return [...new Set(array)].length === array.length;
}

// ✅
function isUnique(array) {
	return new Set(array).size === array.length;
}
```

```js
// ❌
const items = new Set([1, 2, 3, 4, 5]);
if ([...items].length > 3) {
	// do something
}

// ✅
const items = new Set([1, 2, 3, 4, 5]);
if (items.size > 3) {
	// do something
}
```

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

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