setSizeLengthChecks

Prefer Set.size over spreading into an array and accessing .length.

✅ This rule is included in the ts logical presets.

Spreading a Set into an array just to access .length is wasteful. Set has a built-in .size property that provides the same information without creating an intermediate array.

This rule reports when:

• A Set (either new Set() directly or a const variable initialized with a Set) is spread into an array literal
• The array literal immediately accesses .length

Examples

❌ Incorrect

const count = [...new Set(items)].length;

const uniqueItems = new Set([1, 2, 3]);
const count = [...uniqueItems].length;

const count = [...new Set(items)].length;

✅ Correct

const count = new Set(items).size;

const uniqueItems = new Set([1, 2, 3]);
const count = uniqueItems.size;

// Spread is fine when the array is actually used
const uniqueArray = [...new Set(items)];

// Not a Set, so spreading to get length is acceptable
const count = [...items].length;

Options

This rule is not configurable.

When Not To Use It

If you have augmented globals to modify how Array and/or Set and/or ... spreads behave, this rule may not be for you.

Alternately, if you have a large existing codebase where spreading arrays to gain size is the preferred stylistic convention, it may be difficult to migrate to this rule. You might consider using Flint disable comments and/or configuration file disables for those specific situations instead of completely disabling this rule.

Further Reading

• MDN: Set.prototype.size

Equivalents in Other Linters

• ESLint: unicorn/prefer-set-size
• Oxlint: unicorn/prefer-set-size