67 lines
2.0 KiB
Plaintext
67 lines
2.0 KiB
Plaintext
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---
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description: 'Disallow duplicate constituents of union or intersection types.'
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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/no-duplicate-type-constituents** for documentation.
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TypeScript supports types ("constituents") within union and intersection types being duplicates of each other.
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However, developers typically expect each constituent to be unique within its intersection or union.
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Duplicate values make the code overly verbose and generally reduce readability.
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This rule disallows duplicate union or intersection constituents.
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We consider types to be duplicate if they evaluate to the same result in the type system.
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For example, given `type A = string` and `type T = string | A`, this rule would flag that `A` is the same type as `string`.
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<!--tabs-->
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### ❌ Incorrect
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```ts
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type T1 = 'A' | 'A';
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type T2 = A | A | B;
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type T3 = { a: string } & { a: string };
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type T4 = [1, 2, 3] | [1, 2, 3];
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type StringA = string;
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type StringB = string;
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type T5 = StringA | StringB;
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```
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### ✅ Correct
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```ts
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type T1 = 'A' | 'B';
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type T2 = A | B | C;
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type T3 = { a: string } & { b: string };
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type T4 = [1, 2, 3] | [1, 2, 3, 4];
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type StringA = string;
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type NumberB = number;
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type T5 = StringA | NumberB;
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```
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## Options
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### `ignoreIntersections`
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When set to true, duplicate checks on intersection type constituents are ignored.
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### `ignoreUnions`
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When set to true, duplicate checks on union type constituents are ignored.
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## When Not To Use It
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It can sometimes be useful for the sake of documentation to include aliases for the same type.
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You might consider using [ESLint disable comments](https://eslint.org/docs/latest/use/configure/rules#using-configuration-comments-1) for those specific situations instead of completely disabling this rule.
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> In some of those cases, [branded types](https://basarat.gitbook.io/typescript/main-1/nominaltyping#using-interfaces) might be a type-safe way to represent the underlying data types.
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