#prefer-at

#Configuration

import { defineConfig, unicornPlugin } from '@rslint/core';

export default defineConfig([
  unicornPlugin.configs.recommended,
  {
    rules: {
      'unicorn/prefer-at': 'error',
    },
  },
]);

#Rule details

Prefer .at() to read an element or character from an array, typed array, or string. Negative indices count from the end.

The rule checks length-based indices such as items[items.length - 1], one-element reads from .slice(), and calls to last-element helpers such as lodash.last(). It also checks charAt() and one-character substring() calls.

Examples of incorrect code for this rule

const last = items[items.length - 1];
const secondLast = items.slice(-2, -1)[0];
const lastItem = lodash.last(items);
const character = text.charAt(text.length - 1);
const first = text.substring(0, 1);

Examples of correct code for this rule

const last = items.at(-1);
const secondLast = items.at(-2);
const lastItem = items.at(-1);
const character = text.at(-1);
const first = text.at(0);

The rule ignores indexed writes, updates, method calls, and tagged templates because these operations need the original property access. It also ignores indexed reads on arguments and recognized DOM collections, such as element.children and the result of document.querySelectorAll().

#Fixes and suggestions

The rule can autofix length-based reads, one-element .slice() patterns, and last-element helper calls. It offers editor suggestions for string methods so you can check how the replacement handles out-of-range indices. For example, ''.charAt(0) returns an empty string and ''.at(0) returns undefined.

#Options

#checkAllIndexAccess

Defaults to false. Set it to true to check non-negative integer indices that can be determined from the code, such as items[0], and additional charAt() calls. The rule still allows charAt(-1) because it returns an empty string while at(-1) reads from the end.

#getLastElementFunctions

Defaults to []. Add names or dotted paths such as getLast or utils.lastElement for helpers that return the last element of their argument. The rule always recognizes _.last, lodash.last, and underscore.last.

export default [
  {
    plugins: ['unicorn'],
    rules: {
      'unicorn/prefer-at': [
        'error',
        {
          checkAllIndexAccess: true,
          getLastElementFunctions: ['getLast', 'utils.lastElement'],
        },
      ],
    },
  },
];

#Differences from upstream

Rslint withholds autofixes on obvious non-array values such as numbers and plain objects. For example, an object literal can provide numeric keys and a length while lacking an .at() method. The rule reports the pattern and leaves the edit for you.

const items = { 0: 'first', length: 1 };
const last = items[items.length - 1];

Rslint also ignores arguments in slice patterns and through TypeScript assertions. A rest parameter gives you an array with an .at() method.

#Original documentation

• Upstream documentation
• Upstream source