prefer-ts-extras-array-last
Require arrayLast from ts-extras over direct array[array.length - 1] access.
Targeted pattern scopeโ
This rule only matches direct last-element index access (receiver[receiver.length - 1]) where the receiver expression can be reused unchanged in arrayLast(receiver).
- Direct last-element index patterns (
array[array.length - 1]).
Syntactically similar alternatives are intentionally out of scope unless they preserve the same AST shape.
What this rule reportsโ
This rule reports direct receiver[receiver.length - 1] access sites that can be safely replaced with arrayLast(receiver).
- Direct last-element index patterns (
array[array.length - 1]).
Why this rule existsโ
arrayLast makes last-element access explicit and keeps type inference behavior consistent with other ts-extras array helpers.
- Last-element access is easier to audit in large codebases.
- Tuple/readonly array access patterns are standardized.
- Helper-driven access reduces repeated inline index arithmetic.
โ Incorrectโ
const last = values[values.length - 1];
โ Correctโ
const last = arrayLast(values);
Behavior and migration notesโ
- Runtime behavior matches
array[array.length - 1]. - Empty arrays still produce
undefined. - Equivalent index expressions with extra wrappers should be reviewed manually during migration.
Additional examplesโ
โ Incorrect โ Additional exampleโ
const last = rows[rows.length - 1];
โ Correct โ Additional exampleโ
const last = arrayLast(rows);
โ Correct โ Repository-wide usageโ
const lastStep = arrayLast(steps);
ESLint flat config exampleโ
import typefest from "eslint-plugin-typefest";
export default [
{
plugins: { typefest },
rules: {
"typefest/prefer-ts-extras-array-last": "error",
},
},
];
When not to use itโ
Disable this rule if direct index expressions are mandated by local style rules.
Package documentationโ
ts-extras package documentation:
Source file: source/array-last.ts
/**
* Return the last item of an array with stronger typing for tuples.
*
* This provides better type safety than `array[array.length - 1]` or
* `array.at(-1)`.
*
* @category Improved builtin
*
* @example
* ```
* import {arrayLast} from 'ts-extras';
*
* const tuple = ['abc', 123, true] as const;
* const last = arrayLast(tuple);
* //=> true
* // ^? true
*
* const array = ['a', 'b', 'c'];
* const maybeLast = arrayLast(array);
* //=> 'c'
* // ^? string | undefined
*
* // Empty arrays
* const empty: string[] = [];
* const noLast = arrayLast(empty);
* //=> undefined
* // ^? string | undefined
* ```;
*/
Rule catalog ID: R009