# @typescript-eslint/no-for-in-array

> Disallow `for...in` over an array

`for...in` walks the keys of an object, not the items of an array. Over an array, the keys are the indexes as strings, so `i + 1` gives `"01"` and not `1`. The loop also skips holes, and it visits any other enumerable property that the array or its prototype has.

The rule reports arrays, tuples and array-like objects such as a `NodeList`. That covers any type with numeric indexes and a numeric `length`.

`bun lint --type-aware` turns this rule on in TypeScript files.

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

## Configure

Off by default. This rule needs types, so turning it on makes `bun lint` type check your code first.

```toml bunfig.toml
[lint.rules]
"@typescript-eslint/no-for-in-array" = "error" # or "warn"
```

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

## Examples

Reported:

{/* bad */}
```ts
const prices = [5, 10, 20];

for (const i in prices) {
  console.log(i + 1); // "01", "11", "21"
}
```

{/* bad */}
```ts
const links = document.querySelectorAll("a");

for (const key in links) {
  console.log(key); // "0", "1", and also "length", "item", "forEach", ...
}
```

Not reported:

{/* good */}
```ts
const prices = [5, 10, 20];

for (const price of prices) {
  console.log(price);
}

for (const [i, price] of prices.entries()) {
  console.log(i + 1, price);
}
```

{/* good */}
```ts
const stock = { apples: 3, pears: 7 };

for (const name in stock) {
  console.log(name);
}
```