docs: add unique dependencies rule

JoshuaKGoldberg · Nov 12, 2024 · 301e2e4 · 301e2e4
1 parent 19b4c96
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diff --git a/README.md b/README.md
@@ -113,17 +113,17 @@ The default settings don't conflict, and Prettier plugins can quickly fix up ord
 🔧 Automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/user-guide/command-line-interface#--fix).\
 💡 Manually fixable by [editor suggestions](https://eslint.org/docs/latest/use/core-concepts#rule-suggestions).
 
-| Name                                                                   | Description                                                                                     | 💼 | 🔧 | 💡 |
-| :--------------------------------------------------------------------- | :---------------------------------------------------------------------------------------------- | :- | :- | :- |
-| [order-properties](docs/rules/order-properties.md)                     | Package properties must be declared in standard order                                           | ✅  | 🔧 |    |
-| [repository-shorthand](docs/rules/repository-shorthand.md)             | Enforce either object or shorthand declaration for repository.                                  | ✅  | 🔧 |    |
-| [sort-collections](docs/rules/sort-collections.md)                     | Dependencies, scripts, and configuration values must be declared in alphabetical order.         | ✅  | 🔧 |    |
-| [unique-dependencies](docs/rules/unique-dependencies.md)               | Enforce that if repository directory is specified, it matches the path to the package.json file | ✅  |    | 💡 |
-| [valid-local-dependency](docs/rules/valid-local-dependency.md)         | Checks existence of local dependencies in the package.json                                      | ✅  |    |    |
-| [valid-name](docs/rules/valid-name.md)                                 | Enforce that package names are valid npm package names                                          | ✅  |    |    |
-| [valid-package-def](docs/rules/valid-package-def.md)                   | Enforce that package.json has all properties required by the npm spec                           | ✅  |    |    |
-| [valid-repository-directory](docs/rules/valid-repository-directory.md) | Enforce that if repository directory is specified, it matches the path to the package.json file | ✅  |    | 💡 |
-| [valid-version](docs/rules/valid-version.md)                           | Enforce that package versions are valid semver specifiers                                       | ✅  |    |    |
+| Name                                                                   | Description                                                                                       | 💼 | 🔧 | 💡 |
+| :--------------------------------------------------------------------- | :------------------------------------------------------------------------------------------------ | :- | :- | :- |
+| [order-properties](docs/rules/order-properties.md)                     | Package properties must be declared in standard order                                             | ✅  | 🔧 |    |
+| [repository-shorthand](docs/rules/repository-shorthand.md)             | Enforce either object or shorthand declaration for repository.                                    | ✅  | 🔧 |    |
+| [sort-collections](docs/rules/sort-collections.md)                     | Dependencies, scripts, and configuration values must be declared in alphabetical order.           | ✅  | 🔧 |    |
+| [unique-dependencies](docs/rules/unique-dependencies.md)               | Checks a dependency isn't specified more than once (i.e. in `dependencies` and `devDependencies`) | ✅  |    | 💡 |
+| [valid-local-dependency](docs/rules/valid-local-dependency.md)         | Checks existence of local dependencies in the package.json                                        | ✅  |    |    |
+| [valid-name](docs/rules/valid-name.md)                                 | Enforce that package names are valid npm package names                                            | ✅  |    |    |
+| [valid-package-def](docs/rules/valid-package-def.md)                   | Enforce that package.json has all properties required by the npm spec                             | ✅  |    |    |
+| [valid-repository-directory](docs/rules/valid-repository-directory.md) | Enforce that if repository directory is specified, it matches the path to the package.json file   | ✅  |    | 💡 |
+| [valid-version](docs/rules/valid-version.md)                           | Enforce that package versions are valid semver specifiers                                         | ✅  |    |    |
 
 <!-- end auto-generated rules list -->
 <!-- prettier-ignore-end -->

diff --git a/docs/rules/unique-dependencies.md b/docs/rules/unique-dependencies.md
@@ -5,3 +5,28 @@
 💡 This rule is manually fixable by [editor suggestions](https://eslint.org/docs/latest/use/core-concepts#rule-suggestions).
 
 <!-- end auto-generated rule header -->
+
+This rule checks that every dependency is just added once to a [`package.json` key specifying dependencies](/~https://github.com/JoshuaKGoldberg/eslint-plugin-package-json/blob/main/src/rules/unique-dependencies.ts#L8-L16).
+
+Example of **incorrect** code for this rule:
+
+```json
+{
+    "dependencies": {
+        "foo": "1.0.0"
+    },
+    "devDependencies": {
+        "foo": "1.0.0"
+    }
+}
+```
+
+Example of **correct** code for this rule:
+
+```json
+{
+    "dependencies": {
+        "foo": "1.0.0"
+    }
+}
+```
diff --git a/src/rules/unique-dependencies.ts b/src/rules/unique-dependencies.ts
@@ -96,7 +96,7 @@ export const rule = createRule({
 		docs: {
 			category: "Best Practices",
 			description:
-				"Enforce that if repository directory is specified, it matches the path to the package.json file",
+				"Checks a dependency isn't specified more than once (i.e. in `dependencies` and `devDependencies`)",
 			recommended: true,
 		},
 		hasSuggestions: true,