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feat: implement typeConstituentDuplicates for TypeScript plugin #2133

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feat: implement typeConstituentDuplicates for TypeScript plugin #2133

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3 changes: 2 additions & 1 deletion packages/comparisons/src/data.json
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Expand Up @@ -17306,7 +17306,8 @@
"flint": {
"name": "typeConstituentDuplicates",
"plugin": "ts",
"preset": "logical"
"preset": "logical",
"status": "implemented"
},
"oxlint": [
{
Expand Down
71 changes: 71 additions & 0 deletions packages/site/src/content/docs/rules/ts/typeConstituentDuplicates.mdx
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@@ -0,0 +1,71 @@
---
description: "Reports duplicate types in unions and intersections."
title: "typeConstituentDuplicates"
topic: "rules"
---

import { Tabs, TabItem } from "@astrojs/starlight/components";
import { RuleEquivalents } from "~/components/RuleEquivalents";
import RuleSummary from "~/components/RuleSummary.astro";

<RuleSummary plugin="ts" rule="typeConstituentDuplicates" />

Duplicate types in unions and intersections are redundant and can make code harder to read.

## Examples

### Duplicate Union Types

<Tabs>
<TabItem label="❌ Incorrect">

```ts
type Value = string | number | string;
```

</TabItem>
<TabItem label="✅ Correct">

```ts
type Value = string | number;
```

</TabItem>
</Tabs>

### Duplicate Intersection Types

<Tabs>
<TabItem label="❌ Incorrect">

```ts
type Combined = BaseType & ExtendedType & BaseType;
```

</TabItem>
<TabItem label="✅ Correct">

```ts
type Combined = BaseType & ExtendedType;
```

</TabItem>
</Tabs>

## Options

This rule is not configurable.

## When Not To Use It

If you have generated types or complex type manipulations where duplicates are intentional,
you may want to disable this rule.

## Further Reading

- [TypeScript: Union Types](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#union-types)
- [TypeScript: Intersection Types](https://www.typescriptlang.org/docs/handbook/2/objects.html#intersection-types)

## Equivalents in Other Linters

<RuleEquivalents pluginId="ts" ruleId="typeConstituentDuplicates" />
2 changes: 2 additions & 0 deletions packages/ts/src/plugin.ts
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Expand Up @@ -223,6 +223,7 @@ import sequences from "./rules/sequences.ts";
import shadowedRestrictedNames from "./rules/shadowedRestrictedNames.ts";
import sparseArrays from "./rules/sparseArrays.ts";
import symbolDescriptions from "./rules/symbolDescriptions.ts";
import typeConstituentDuplicates from "./rules/typeConstituentDuplicates.ts";
import typeofComparisons from "./rules/typeofComparisons.ts";
import unassignedVariables from "./rules/unassignedVariables.ts";
import undefinedVariables from "./rules/undefinedVariables.ts";
Expand Down Expand Up @@ -465,6 +466,7 @@ export const ts = createPlugin({
shadowedRestrictedNames,
sparseArrays,
symbolDescriptions,
typeConstituentDuplicates,
typeofComparisons,
unassignedVariables,
undefinedVariables,
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52 changes: 52 additions & 0 deletions packages/ts/src/rules/typeConstituentDuplicates.test.ts
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import rule from "./typeConstituentDuplicates.ts";
import { ruleTester } from "./ruleTester.ts";

Check failure on line 2 in packages/ts/src/rules/typeConstituentDuplicates.test.ts

View workflow job for this annotation

GitHub Actions / Lint 

Expected "./ruleTester.ts" to come before "./typeConstituentDuplicates.ts"

ruleTester.describe(rule, {
invalid: [
{
code: `type Value = string | string;`,
snapshot: `type Value = string | string;
~~~~~~
Duplicate type in union.`,
},
{
code: `type Value = 1 | 2 | 1;`,
snapshot: `type Value = 1 | 2 | 1;
~
Duplicate type in union.`,
},
{
code: `type Value = "a" | "b" | "a";`,
snapshot: `type Value = "a" | "b" | "a";
~~~
Duplicate type in union.`,
},
{
code: `type Value = A & B & A;`,
snapshot: `type Value = A & B & A;
~
Duplicate type in intersection.`,
},
{
code: `type Value = { name: string } & { name: string };`,
snapshot: `type Value = { name: string } & { name: string };
~~~~~~~~~~~~~~~~
Duplicate type in intersection.`,
},
{
code: `type Value = Array<string> | Array<string>;`,
snapshot: `type Value = Array<string> | Array<string>;
~~~~~~~~~~~~~
Duplicate type in union.`,
},
],
valid: [
`type Value = string | number;`,
`type Value = 1 | 2 | 3;`,
`type Value = "a" | "b" | "c";`,
`type Value = A & B & C;`,
`type Value = { name: string } & { age: number };`,
`type Value = Array<string> | Array<number>;`,
`type Value = string;`,
],
});
60 changes: 60 additions & 0 deletions packages/ts/src/rules/typeConstituentDuplicates.ts
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import { typescriptLanguage } from "@flint.fyi/typescript-language";
import type ts from "typescript";

import { ruleCreator } from "./ruleCreator.ts";

export default ruleCreator.createRule(typescriptLanguage, {
about: {
description: "Reports duplicate types in unions and intersections.",
id: "typeConstituentDuplicates",
presets: ["logical"],
},
messages: {
duplicateIntersection: {
primary: "Duplicate type in intersection.",
secondary: ["This type appears more than once in the intersection."],
suggestions: ["Remove the duplicate type."],
},
duplicateUnion: {
primary: "Duplicate type in union.",
secondary: ["This type appears more than once in the union."],
suggestions: ["Remove the duplicate type."],
},
},
setup(context) {
function checkDuplicates(
types: ts.NodeArray<ts.TypeNode>,
sourceFile: ts.SourceFile,
messageId: "duplicateIntersection" | "duplicateUnion",
) {
const seen = new Map<string, ts.TypeNode>();

for (const typeNode of types) {
const text = typeNode.getText(sourceFile);

if (seen.has(text)) {
context.report({
message: messageId,
range: {
begin: typeNode.getStart(sourceFile),
end: typeNode.getEnd(),
},
});
} else {
seen.set(text, typeNode);
}
}
}

return {
visitors: {
IntersectionType(node, { sourceFile }) {
checkDuplicates(node.types, sourceFile, "duplicateIntersection");
},
UnionType(node, { sourceFile }) {
checkDuplicates(node.types, sourceFile, "duplicateUnion");
},
},
};
},
});
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