array-type

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

🎨

Extending "plugin:@typescript-eslint/stylistic" in an ESLint configuration enables this rule.

🔧

Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.

TypeScript provides two equivalent ways to define an array type: T[] and Array<T>. The two styles are functionally equivalent. Using the same style consistently across your codebase makes it easier for developers to read and understand array types.

.eslintrc.cjs

module.exports = {
  "rules": {
    "@typescript-eslint/array-type": "error"
  }
};

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Options

This rule accepts the following options:

type ArrayOption = 'array' | 'array-simple' | 'generic';

type Options = [
  {
    /** The array type expected for mutable cases. */
    default?: ArrayOption;
    /** The array type expected for readonly cases. If omitted, the value for `default` will be used. */
    readonly?: ArrayOption;
  },
];

const defaultOptions: Options = [{ default: 'array' }];

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'];

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✅ Correct

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

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"generic"

Always use Array<T>, ReadonlyArray<T>, or Readonly<Array<T>> for all array types. readonly T[] will be modified to ReadonlyArray<T> and Readonly<T[]> will be modified to Readonly<Array<T>.

❌ Incorrect

const x: string[] = ['a', 'b'];
const y: readonly string[] = ['a', 'b'];
const z: Readonly<string[]> = ['a', 'b'];

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✅ Correct

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

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"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'];

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✅ 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'];

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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>

When Not To Use It

This rule is purely a stylistic rule for maintaining consistency in your project. You can turn it off if you don't want to keep a consistent style for array types.

However, keep in mind that inconsistent style can harm readability in a project. We recommend picking a single option for this rule that works best for your project.

Resources

• Rule source
• Test source