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no-duplicate-type-constituents

Disallow duplicate constituents of union or intersection types.

✅

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

🔧

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

💭

This rule requires type information to run.

TypeScript supports types ("constituents") within union and intersection types being duplicates of each other. However, developers typically expect each constituent to be unique within its intersection or union. Duplicate values make the code overly verbose and generally reduce readability.

This rule disallows duplicate union or intersection constituents. We consider types to be duplicate if they evaluate to the same result in the type system. For example, given type A = string and type T = string | A, this rule would flag that A is the same type as string.

This rule also disallows explicitly listing undefined in a type union when a function parameter is marked as optional. Doing so is unnecessary. Please note that this check only applies to parameters, not properties. Therefore, it does not conflict with the exactOptionalPropertyTypes TypeScript compiler setting.

type T1 = 'A' | 'A';

type T2 = string | string | number;

type T3 = { a: string } & { a: string };

type T4 = [1, 2, 3] | [1, 2, 3];

type StringA = string;
type StringB = string;
type T5 = StringA | StringB;

const fn = (a?: string | undefined) => {};
Open in Playground
eslint.config.mjs
export default tseslint.config({
  rules: {
    "@typescript-eslint/no-duplicate-type-constituents": "error"
  }
});

Try this rule in the playground ↗

Options​

This rule accepts the following options:

type Options = [
  {
    /** Whether to ignore `&` intersections. */
    ignoreIntersections?: boolean;
    /** Whether to ignore `|` unions. */
    ignoreUnions?: boolean;
  },
];

const defaultOptions: Options = [
  { ignoreIntersections: false, ignoreUnions: false },
];

ignoreIntersections​

Whether to ignore & intersections. Default: false.

When set to true, duplicate checks on intersection type constituents are ignored.

ignoreUnions​

Whether to ignore | unions. Default: false.

When set to true, duplicate checks on union type constituents are ignored.

When Not To Use It​

It can sometimes be useful for the sake of documentation to include aliases for the same type. You might consider using ESLint disable comments for those specific situations instead of completely disabling this rule.

In some of those cases, branded types might be a type-safe way to represent the underlying data types.

Type checked lint rules are more powerful than traditional lint rules, but also require configuring type checked linting.

See Troubleshooting > Linting with Type Information > Performance if you experience performance degradations after enabling type checked rules.

Resources​