# unicorn/prefer-at

> Require `.at()` to read an element counted from the end of an array or string

`.at(-1)` reads the last element. Without it, you have to name the array twice, as in `items[items.length - 1]`, or copy part of it with `.slice(-1)[0]`.

Taking one character with `.substring(index, index + 1)` is reported too.

`--fix` rewrites these to `.at()`, except `.substring()`, which treats an index outside the string differently. `--fix-suggestions` rewrites that one.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

Off by default. To turn it on:

```toml bunfig.toml
[lint.rules]
"unicorn/prefer-at" = "error" # or "warn"
```

Every option, set to its default:

```toml bunfig.toml
[lint.rules]
"unicorn/prefer-at" = ["error", {
  checkAllIndexAccess = false,
  getLastElementFunctions = [], # list of strings
}]
```

{/* End of the generated section. */}

## Examples

Reported:

{/* bad */}
```js
const last = items[items.length - 1];
const secondToLast = items.slice(-2)[0];
const tail = items.slice(-1).pop();
const newest = _.last(items);
const end = name.charAt(name.length - 1);
const letter = name.substring(index, index + 1);
```

Not reported:

{/* good */}
```js
const last = items.at(-1);
const secondToLast = items.at(-2);
const end = name.at(-1);
const letter = name.at(index);
const first = items[0];
```

`.at()` only reads, so writing to an element isn't reported:

{/* good */}
```js
items[items.length - 1] = value;
```

## Options

| Option | Default | What it does |
| --- | --- | --- |
| `checkAllIndexAccess` | `false` | Also report an index counted from the start, such as `items[0]` and `name.charAt(2)`. |
| `getLastElementFunctions` | `[]` | Other objects with a `last()` function to report, besides `_`, `lodash` and `underscore`. |

{/* bad {"options": [{"checkAllIndexAccess": true}], "count": 2} */}
```js
const first = items[0];
const third = name.charAt(2);
```

{/* bad {"options": [{"getLastElementFunctions": ["utils"]}]} */}
```js
const newest = utils.last(items);
```

## When not to use it

`.at()` returns `undefined` past the end of a string, where `.charAt()` and `.substring()` return `""`. Check code that relies on the empty string before you change it.