array-type

Requires using either T[] or Array<T> for arrays.

Using the same style for array definitions across your codebase makes it easier for your developers to read and understand the types.

Attributes

• Included in configs
  • ✅ Recommended
  • 🔒 Strict
• Fixable
  • 🔧 Automated Fixer
  • 🛠 Suggestion Fixer
• 💭 Requires type information

Rule Details

This rule aims to standardize usage of array types within your codebase.

Options

type ArrayOption = 'array' | 'generic' | 'array-simple';
type Options = {
  default: ArrayOption;
  readonly?: ArrayOption;
};

const defaultOptions: Options = {
  default: 'array',
};

The rule accepts an options object with the following properties:

• default - sets the array type expected for mutable cases.
• readonly - sets the array type expected for readonly arrays. If this is omitted, then the value for default will be used.

Each property can be set to one of three strings: 'array' | 'generic' | 'array-simple'.

The default config will enforce that all mutable and readonly arrays use the 'array' syntax.

"array"

Always use T[] or readonly T[] for all array types.

❌ Incorrect

const x: Array<string> = ['a', 'b'];
const y: ReadonlyArray<string> = ['a', 'b'];

✅ Correct

const x: string[] = ['a', 'b'];
const y: readonly string[] = ['a', 'b'];

"generic"

Always use Array<T> or ReadonlyArray<T> for all array types.

❌ Incorrect

const x: string[] = ['a', 'b'];
const y: readonly string[] = ['a', 'b'];

✅ Correct

const x: Array<string> = ['a', 'b'];
const y: ReadonlyArray<string> = ['a', 'b'];

"array-simple"

Use T[] or readonly T[] for simple types (i.e. types which are just primitive names or type references). Use Array<T> or ReadonlyArray<T> for all other types (union types, intersection types, object types, function types, etc).

❌ Incorrect

const a: (string | number)[] = ['a', 'b'];
const b: { prop: string }[] = [{ prop: 'a' }];
const c: (() => void)[] = [() => {}];
const d: Array<MyType> = ['a', 'b'];
const e: Array<string> = ['a', 'b'];
const f: ReadonlyArray<string> = ['a', 'b'];

✅ Correct

const a: Array<string | number> = ['a', 'b'];
const b: Array<{ prop: string }> = [{ prop: 'a' }];
const c: Array<() => void> = [() => {}];
const d: MyType[] = ['a', 'b'];
const e: string[] = ['a', 'b'];
const f: readonly string[] = ['a', 'b'];

Combination Matrix

This matrix lists all possible option combinations and their expected results for different types of Arrays.

defaultOption	readonlyOption	Array with simple type	Array with non simple type	Readonly array with simple type	Readonly array with non simple type
array		number[]	(Foo & Bar)[]	readonly number[]	readonly (Foo & Bar)[]
array	array	number[]	(Foo & Bar)[]	readonly number[]	readonly (Foo & Bar)[]
array	array-simple	number[]	(Foo & Bar)[]	readonly number[]	ReadonlyArray<Foo & Bar>
array	generic	number[]	(Foo & Bar)[]	ReadonlyArray<number>	ReadonlyArray<Foo & Bar>
array-simple		number[]	Array<Foo & Bar>	readonly number[]	ReadonlyArray<Foo & Bar>
array-simple	array	number[]	Array<Foo & Bar>	readonly number[]	readonly (Foo & Bar)[]
array-simple	array-simple	number[]	Array<Foo & Bar>	readonly number[]	ReadonlyArray<Foo & Bar>
array-simple	generic	number[]	Array<Foo & Bar>	ReadonlyArray<number>	ReadonlyArray<Foo & Bar>
generic		Array<number>	Array<Foo & Bar>	ReadonlyArray<number>	ReadonlyArray<Foo & Bar>
generic	array	Array<number>	Array<Foo & Bar>	readonly number[]	readonly (Foo & Bar)[]
generic	array-simple	Array<number>	Array<Foo & Bar>	readonly number[]	ReadonlyArray<Foo & Bar>
generic	generic	Array<number>	Array<Foo & Bar>	ReadonlyArray<number>	ReadonlyArray<Foo & Bar>