# unicorn/prefer-negative-index

> Require a negative index instead of `.length - index`

`slice`, `at`, `splice`, `toSpliced` and `subarray` count a negative index from the end, so `items.slice(items.length - 2)` can be `items.slice(-2)`.

The rule only looks at the index arguments. That's the first two for `slice` and `subarray`, and the first for the others.

`--fix` removes the `.length` part.

{/* 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-negative-index" = "error" # or "warn"
```

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

## Examples

Reported:

{/* bad {"count": 4} */}
```js
const lastTwo = items.slice(items.length - 2);
const last = items.at(items.length - 1);
queue.splice(queue.length - 1, 1);
const rest = Array.prototype.slice.call(list, list.length - 2);
```

Not reported:

{/* good */}
```js
const lastTwo = items.slice(-2);
const last = items.at(-1);
queue.splice(-1, 1);
const rest = Array.prototype.slice.call(list, -2);
```

The length must come from the same object, minus a number literal. Bracket access isn't reported, because `items[-1]` is `undefined`:

{/* good */}
```js
const rest = items.slice(other.length - 2);
const tail = items.slice(items.length - count); // slice(-count) is the whole array when count is 0
const last = items[items.length - 1];
```