# unicorn/no-useless-iterator-to-array

> Disallow `.toArray()` on an iterator where the iterator itself works

`iterator.toArray()` walks the whole iterator and allocates an array. The array is wasted if the next step takes any iterable, or calls a method that iterators have too.

The rule reports `.toArray()`:

- in `for...of`, `yield*` and a spread into an array or a call
- as the argument of `new Set()`, `new Map()`, `new WeakSet()`, `new WeakMap()` or a typed array constructor
- as the argument of `Array.from()`, `Object.fromEntries()`, `Promise.all()`, `Promise.allSettled()`, `Promise.any()` or `Promise.race()`
- before `.every()`, `.find()`, `.forEach()`, `.some()` or `.reduce()` with an initial value

`--fix` removes the `.toArray()`. Before an iterator method and in the `Promise` methods, only `--fix-suggestions` does.

{/* 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/no-useless-iterator-to-array" = "error" # or "warn"
```

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

## Examples

Reported:

{/* bad {"count": 4} */}
```js
for (const name of users.keys().toArray()) {
  console.log(name);
}

const ids = new Set(users.values().map(user => user.id).toArray());
const list = [...users.values().toArray()];
const hasAdmin = users.values().toArray().some(user => user.admin);
```

Not reported:

{/* good */}
```js
for (const name of users.keys()) {
  console.log(name);
}

const ids = new Set(users.values().map(user => user.id));
const list = [...users.values()];
const hasAdmin = users.values().some(user => user.admin);
```

Methods that only arrays have still need the array, and so does a callback that takes the array as its third parameter:

{/* good */}
```js
const sorted = users.values().toArray().sort(byName);

const isUnique = users.values().toArray().every((user, index, all) => all.indexOf(user) === index);
```

## When not to use it

The rule can't see types, so it reports every `.toArray()` call without arguments in these places. Turn it off if you use a library with its own `toArray()` on objects that aren't iterable.