# prefer-type-fest-unknown-array

Require TypeFest [`UnknownArray`](https://github.com/sindresorhus/type-fest/blob/main/source/unknown-array.d.ts) over `readonly unknown[]` and `ReadonlyArray<unknown>`.

## Targeted pattern scope

This rule targets explicit unknown-array spellings that TypeFest standardizes as `UnknownArray`.

- `readonly unknown[]`
- `ReadonlyArray<unknown>`

Other collection contracts are left alone unless they match the exact unknown collection form listed below.

## What this rule reports

This rule reports unknown-array type forms that should migrate to `UnknownArray`.

- `readonly unknown[]`
- `ReadonlyArray<unknown>`

## Why this rule exists

`UnknownArray` provides a clearer, shared alias for unknown element arrays and keeps utility-type usage consistent with other TypeFest-first conventions.

## ❌ Incorrect

```ts
type Values = readonly unknown[];
```

## ✅ Correct

```ts
type Values = UnknownArray;
```

## Behavior and migration notes

- `UnknownArray` is the canonical alias for readonly unknown-element arrays in `type-fest` style.
- This rule normalizes `readonly unknown[]` and `ReadonlyArray<unknown>` into one shared name.
- Use this alias for untyped collection ingress points before narrowing.

## Additional examples

### ❌ Incorrect — Additional example

```ts
type Input = readonly unknown[];
```

### ✅ Correct — Additional example

```ts
type Input = UnknownArray;
```

### ✅ Correct — Repository-wide usage

```ts
type PayloadList = UnknownArray;
```

## ESLint flat config example

```ts
import typefest from "eslint-plugin-typefest";

export default [
 {
  plugins: { typefest },
  rules: {
   "typefest/prefer-type-fest-unknown-array": "error",
  },
 },
];
```

## When not to use it

Disable this rule if external API signatures must preserve existing alias names.

## Package documentation

TypeFest package documentation:

Source file: [`source/unknown-array.d.ts`](https://github.com/sindresorhus/type-fest/blob/main/source/unknown-array.d.ts)

````ts
/**
 * Represents an array with `unknown` value.
 *
 * Use case: You want a type that all arrays can be assigned to, but you don't
 * care about the value.
 *
 * @category Type
 * @category Array
 *
 * @example
 *  ```
 *  import type {UnknownArray} from 'type-fest';
 *
 *  type IsArray<T> = T extends UnknownArray ? true : false;
 *
 *  type A = IsArray<['foo']>;
 *  //=> true
 *
 *  type B = IsArray<readonly number[]>;
 *  //=> true
 *
 *  type C = IsArray<string>;
 *  //=> false
 *  ```;
 */
````

> **Rule catalog ID:** R069

## Further reading

- [`type-fest` README](https://github.com/sindresorhus/type-fest)
- [`type-fest` npm documentation](https://www.npmjs.com/package/type-fest)
- [TypeScript Handbook: Utility Types](https://www.typescriptlang.org/docs/handbook/utility-types.html)

## Adoption resources

- [Rule adoption checklist](./guides/adoption-checklist.md)
- [Rollout and fix safety](./guides/rollout-and-fix-safety.md)