112 lines
4.9 KiB
Plaintext
112 lines
4.9 KiB
Plaintext
---
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description: 'Require consistently using either `T[]` or `Array<T>` for arrays.'
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---
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> 🛑 This file is source code, not the primary documentation location! 🛑
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>
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> See **https://typescript-eslint.io/rules/array-type** for documentation.
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TypeScript provides two equivalent ways to define an array type: `T[]` and `Array<T>`.
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The two styles are functionally equivalent.
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Using the same style consistently across your codebase makes it easier for developers to read and understand array types.
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## Options
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The default config will enforce that all mutable and readonly arrays use the `'array'` syntax.
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### `"array"`
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Always use `T[]` or `readonly T[]` for all array types.
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<!--tabs-->
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#### ❌ Incorrect
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```ts option='{ "default": "array" }'
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const x: Array<string> = ['a', 'b'];
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const y: ReadonlyArray<string> = ['a', 'b'];
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```
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#### ✅ Correct
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```ts option='{ "default": "array" }'
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const x: string[] = ['a', 'b'];
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const y: readonly string[] = ['a', 'b'];
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```
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### `"generic"`
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Always use `Array<T>` or `ReadonlyArray<T>` for all array types.
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<!--tabs-->
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#### ❌ Incorrect
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```ts option='{ "default": "generic" }'
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const x: string[] = ['a', 'b'];
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const y: readonly string[] = ['a', 'b'];
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```
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#### ✅ Correct
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```ts option='{ "default": "generic" }'
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const x: Array<string> = ['a', 'b'];
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const y: ReadonlyArray<string> = ['a', 'b'];
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```
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### `"array-simple"`
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Use `T[]` or `readonly T[]` for simple types (i.e. types which are just primitive names or type references).
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Use `Array<T>` or `ReadonlyArray<T>` for all other types (union types, intersection types, object types, function types, etc).
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<!--tabs-->
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#### ❌ Incorrect
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```ts option='{ "default": "array-simple" }'
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const a: (string | number)[] = ['a', 'b'];
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const b: { prop: string }[] = [{ prop: 'a' }];
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const c: (() => void)[] = [() => {}];
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const d: Array<MyType> = ['a', 'b'];
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const e: Array<string> = ['a', 'b'];
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const f: ReadonlyArray<string> = ['a', 'b'];
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```
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#### ✅ Correct
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```ts option='{ "default": "array-simple" }'
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const a: Array<string | number> = ['a', 'b'];
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const b: Array<{ prop: string }> = [{ prop: 'a' }];
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const c: Array<() => void> = [() => {}];
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const d: MyType[] = ['a', 'b'];
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const e: string[] = ['a', 'b'];
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const f: readonly string[] = ['a', 'b'];
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```
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## Combination Matrix
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This matrix lists all possible option combinations and their expected results for different types of Arrays.
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| defaultOption | readonlyOption | Array with simple type | Array with non simple type | Readonly array with simple type | Readonly array with non simple type |
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| -------------- | -------------- | ---------------------- | -------------------------- | ------------------------------- | ----------------------------------- |
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| `array` | | `number[]` | `(Foo & Bar)[]` | `readonly number[]` | `readonly (Foo & Bar)[]` |
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| `array` | `array` | `number[]` | `(Foo & Bar)[]` | `readonly number[]` | `readonly (Foo & Bar)[]` |
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| `array` | `array-simple` | `number[]` | `(Foo & Bar)[]` | `readonly number[]` | `ReadonlyArray<Foo & Bar>` |
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| `array` | `generic` | `number[]` | `(Foo & Bar)[]` | `ReadonlyArray<number>` | `ReadonlyArray<Foo & Bar>` |
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| `array-simple` | | `number[]` | `Array<Foo & Bar>` | `readonly number[]` | `ReadonlyArray<Foo & Bar>` |
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| `array-simple` | `array` | `number[]` | `Array<Foo & Bar>` | `readonly number[]` | `readonly (Foo & Bar)[]` |
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| `array-simple` | `array-simple` | `number[]` | `Array<Foo & Bar>` | `readonly number[]` | `ReadonlyArray<Foo & Bar>` |
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| `array-simple` | `generic` | `number[]` | `Array<Foo & Bar>` | `ReadonlyArray<number>` | `ReadonlyArray<Foo & Bar>` |
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| `generic` | | `Array<number>` | `Array<Foo & Bar>` | `ReadonlyArray<number>` | `ReadonlyArray<Foo & Bar>` |
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| `generic` | `array` | `Array<number>` | `Array<Foo & Bar>` | `readonly number[]` | `readonly (Foo & Bar)[]` |
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| `generic` | `array-simple` | `Array<number>` | `Array<Foo & Bar>` | `readonly number[]` | `ReadonlyArray<Foo & Bar>` |
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| `generic` | `generic` | `Array<number>` | `Array<Foo & Bar>` | `ReadonlyArray<number>` | `ReadonlyArray<Foo & Bar>` |
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## When Not To Use It
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This rule is purely a stylistic rule for maintaining consistency in your project.
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You can turn it off if you don't want to keep a consistent style for array types.
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However, keep in mind that inconsistent style can harm readability in a project.
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We recommend picking a single option for this rule that works best for your project.
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