setHasExistenceChecks

Prefer Set.has() over Array.includes() for repeated existence checks.

✅ This rule is included in the ts stylisticStrict presets.

When an array is only used for existence checks via .includes(), converting it to a Set and using .has() provides better performance. Array.includes() has O(n) time complexity, while Set.has() has O(1) time complexity.

This rule reports when:

• A const variable is initialized with an array (literal, Array(), Array.from(), Array.of(), or array methods like .filter(), .map(), etc.)
• All usages of that variable are .includes() calls with exactly one argument
• Either there are multiple .includes() calls, or a single call is inside a loop or function

Examples

❌ Incorrect

const items = [1, 2, 3];
function check(value: number) {
  return items.includes(value);
}

const items = [1, 2, 3];
items.includes(1);
items.includes(2);

const items = [1, 2, 3];
for (const value of values) {
  items.includes(value);
}

✅ Correct

const items = new Set([1, 2, 3]);
function check(value: number) {
  return items.has(value);
}

const items = new Set([1, 2, 3]);
items.has(1);
items.has(2);

// Single call, not in loop/function - acceptable
const items = [1, 2, 3];
items.includes(1);

// Has other usages besides .includes()
const items = [1, 2, 3];
items.push(4);
items.includes(1);

// Exported arrays are not reported
export const items = [1, 2, 3];
items.includes(1);
items.includes(2);

Options

This rule is not configurable.

When Not To Use It

If the array is very small (a few elements), the performance difference between Array.includes() and Set.has() is negligible. You may also disable this rule if you need to preserve the array type for compatibility with APIs that expect arrays.

Further Reading

• MDN: Set.prototype.has()
• MDN: Array.prototype.includes()

Equivalents in Other Linters

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