@studnicky/prefer-collection-types

Flags four patterns where arrays or plain objects perform worse than Set or Map:

• Pattern A: Inline array literal .includes() — ['a', 'b'].includes(x) should use new Set(['a', 'b']).has(x).
• Pattern B: Object.fromEntries() accessed via computed bracket — use new Map(pairs).get(key).
• Pattern C: Module-scope const arrays used exclusively for .includes() membership tests — declare as new Set(...).
• Pattern D: Array literal .includes() inside .filter(), .some(), .every(), .find(), or .findIndex() callbacks — convert the array to a Set.

Set.has is 29× faster than Array.includes on equal-size inputs.

Fixable: No · Options: checkArrayLiterals, checkFromEntries, checkModuleScopeArrays · Suggested severity: warn

✗ Incorrect

// Pattern A — inline array .includes()
if (['admin', 'moderator', 'owner'].includes(role)) { /* ... */ }

// Pattern C — module-scope array used only for .includes()
const VALID_METHODS = ['GET', 'POST', 'PUT', 'DELETE'];
if (VALID_METHODS.includes(method)) { /* ... */ }

// Pattern D — .includes() inside iteration callback
const active = users.filter(u => ['active', 'pending'].includes(u.status));

// Pattern B — Object.fromEntries accessed via computed key
const value = Object.fromEntries(pairs)[key];

✓ Correct

// Use Set for membership tests
if (new Set(['admin', 'moderator', 'owner']).has(role)) { /* ... */ }

// Module-scope Set
const VALID_METHODS = new Set(['GET', 'POST', 'PUT', 'DELETE']);
if (VALID_METHODS.has(method)) { /* ... */ }

// Set inside iteration callback
const ACTIVE_STATUSES = new Set(['active', 'pending']);
const active = users.filter(u => ACTIVE_STATUSES.has(u.status));

// Use Map instead of fromEntries + bracket access
const map = new Map(pairs);
const value = map.get(key);

Options

{
  "@studnicky/prefer-collection-types": ["warn", {
    "checkArrayLiterals": true,
    "checkFromEntries": true,
    "checkModuleScopeArrays": true
  }]
}

Option	Type	Default	Description
checkArrayLiterals	boolean	true	Flag inline array literals used with .includes() (Patterns A and D).
checkFromEntries	boolean	true	Flag Object.fromEntries() results accessed with computed bracket notation (Pattern B).
checkModuleScopeArrays	boolean	true	Flag module-scope const arrays used exclusively for .includes() membership tests (Pattern C).