TypeScript · performance

prefer-module-level-constant

eslint:prefer-module-level-constant

Hoist literal-only constant collections and regexes out of function bodies to module scope so they are allocated once.

Default: error
Fix: none
Languages: typescript

Why

Recreating immutable lookup data on every call wastes allocations and obscures its constant nature.

Fix

Declare immutable literal collections and non-stateful regular expressions once at module scope.

Before / after

Executed by this rule’s unit tests.

Before

Do not recreate a constant collection

src/keys.ts · focus

function isAllowed(key: string) { const KEYS = ['a', 'b', 'c']; return KEYS.includes(key); }

After

Hoist a constant collection

src/keys.ts · focus

const KEYS = ['a', 'b', 'c'] as const; function isAllowed(key: string) { return KEYS.includes(key); }

Limits

Collections that are small, mutated, escape the function, or depend on local values are not reported.

Message IDs

hoistCollectionhoistRegex

Options

{
  "additionalProperties": false,
  "properties": {
    "checkRegex": {
      "type": "boolean"
    },
    "ignoreTestFiles": {
      "type": "boolean"
    },
    "minElements": {
      "minimum": 1,
      "type": "number"
    }
  },
  "type": "object"
}