typescript-eslint · JoshuaKGoldberg · Apr 27, 2022 · Apr 4, 2022 · Apr 17, 2022 · Apr 23, 2022
diff --git a/packages/eslint-plugin/README.md b/packages/eslint-plugin/README.md
@@ -119,6 +119,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
 | [`@typescript-eslint/no-base-to-string`](./docs/rules/no-base-to-string.md)                                             | Requires that `.toString()` is only called on objects which provide useful information when stringified                 |                    |          | :thought_balloon: |
 | [`@typescript-eslint/no-confusing-non-null-assertion`](./docs/rules/no-confusing-non-null-assertion.md)                 | Disallow non-null assertion in locations that may be confusing                                                          |                    | :wrench: |                   |
 | [`@typescript-eslint/no-confusing-void-expression`](./docs/rules/no-confusing-void-expression.md)                       | Requires expressions of type void to appear in statement position                                                       |                    | :wrench: | :thought_balloon: |
+| [`@typescript-eslint/no-duplicate-enum-values`](./docs/rules/no-duplicate-enum-values.md)                               | Disallow duplicate enum member values                                                                                   |                    |          |                   |
 | [`@typescript-eslint/no-dynamic-delete`](./docs/rules/no-dynamic-delete.md)                                             | Disallow the delete operator with computed key expressions                                                              |                    | :wrench: |                   |
 | [`@typescript-eslint/no-empty-interface`](./docs/rules/no-empty-interface.md)                                           | Disallow the declaration of empty interfaces                                                                            | :white_check_mark: | :wrench: |                   |
 | [`@typescript-eslint/no-explicit-any`](./docs/rules/no-explicit-any.md)                                                 | Disallow usage of the `any` type                                                                                        | :white_check_mark: | :wrench: |                   |

diff --git a/packages/eslint-plugin/docs/rules/no-duplicate-enum-values.md b/packages/eslint-plugin/docs/rules/no-duplicate-enum-values.md
@@ -0,0 +1,51 @@
+# `no-duplicate-enum-values`
+
+Disallow duplicate enum member values.
+
+Although TypeScript supports duplicate enum member values, people usually expect members to have unique values within the same enum. Duplicate values can lead to bugs that are hard to track down.
+
+## Rule Details
+
+This rule disallows defining an enum with multiple members initialized to the same value. Now it only enforces on enum members initialized with String or Number literals. Members without initializer or initialized with an expression are not checked by this rule.
+
+<!--tabs-->
+
+### ❌ Incorrect
+
+```ts
+enum E {
+  A = 0,
+  B = 0,
+}
+```
+
+```ts
+enum E {
+  A = 'A'
+  B = 'A'
+}
+```
+
+### ✅ Correct
+
+```ts
+enum E {
+  A = 0,
+  B = 1,
+}
+```
+
+```ts
+enum E {
+  A = 'A'
+  B = 'B'
+}
+```
+
+This rule is not configurable.
+
+## Attributes
+
+- [ ] ✅ Recommended
+- [ ] 🔧 Fixable
+- [ ] 💭 Requires type information
diff --git a/packages/eslint-plugin/src/configs/all.ts b/packages/eslint-plugin/src/configs/all.ts
@@ -21,8 +21,8 @@ export = {
     '@typescript-eslint/consistent-indexed-object-style': 'error',
     '@typescript-eslint/consistent-type-assertions': 'error',
     '@typescript-eslint/consistent-type-definitions': 'error',
-    '@typescript-eslint/consistent-type-imports': 'error',
     '@typescript-eslint/consistent-type-exports': 'error',
+    '@typescript-eslint/consistent-type-imports': 'error',
     'default-param-last': 'off',
     '@typescript-eslint/default-param-last': 'error',
     'dot-notation': 'off',
@@ -51,6 +51,7 @@ export = {
     '@typescript-eslint/no-confusing-void-expression': 'error',
     'no-dupe-class-members': 'off',
     '@typescript-eslint/no-dupe-class-members': 'error',
+    '@typescript-eslint/no-duplicate-enum-values': 'error',
     'no-duplicate-imports': 'off',
     '@typescript-eslint/no-duplicate-imports': 'error',
     '@typescript-eslint/no-dynamic-delete': 'error',
@@ -115,9 +116,9 @@ export = {
     '@typescript-eslint/no-unused-vars': 'error',
     'no-use-before-define': 'off',
     '@typescript-eslint/no-use-before-define': 'error',
-    '@typescript-eslint/no-useless-empty-export': 'error',
     'no-useless-constructor': 'off',
     '@typescript-eslint/no-useless-constructor': 'error',
+    '@typescript-eslint/no-useless-empty-export': 'error',
     '@typescript-eslint/no-var-requires': 'error',
     '@typescript-eslint/non-nullable-type-assertion-style': 'error',
     'object-curly-spacing': 'off',
@@ -153,12 +154,12 @@ export = {
     semi: 'off',
     '@typescript-eslint/semi': 'error',
     '@typescript-eslint/sort-type-union-intersection-members': 'error',
+    'space-before-blocks': 'off',
+    '@typescript-eslint/space-before-blocks': 'error',
     'space-before-function-paren': 'off',
     '@typescript-eslint/space-before-function-paren': 'error',
     'space-infix-ops': 'off',
     '@typescript-eslint/space-infix-ops': 'error',
-    'space-before-blocks': 'off',
-    '@typescript-eslint/space-before-blocks': 'error',
     '@typescript-eslint/strict-boolean-expressions': 'error',
     '@typescript-eslint/switch-exhaustiveness-check': 'error',
     '@typescript-eslint/triple-slash-reference': 'error',

diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
@@ -32,6 +32,7 @@ import noBaseToString from './no-base-to-string';
 import confusingNonNullAssertionLikeNotEqual from './no-confusing-non-null-assertion';
 import noConfusingVoidExpression from './no-confusing-void-expression';
 import noDupeClassMembers from './no-dupe-class-members';
+import noDuplicateEnumValues from './no-duplicate-enum-values';
 import noDuplicateImports from './no-duplicate-imports';
 import noDynamicDelete from './no-dynamic-delete';
 import noEmptyFunction from './no-empty-function';
@@ -159,6 +160,7 @@ export default {
   'no-confusing-non-null-assertion': confusingNonNullAssertionLikeNotEqual,
   'no-confusing-void-expression': noConfusingVoidExpression,
   'no-dupe-class-members': noDupeClassMembers,
+  'no-duplicate-enum-values': noDuplicateEnumValues,
   'no-duplicate-imports': noDuplicateImports,
   'no-dynamic-delete': noDynamicDelete,
   'no-empty-function': noEmptyFunction,

diff --git a/packages/eslint-plugin/src/rules/no-duplicate-enum-values.ts b/packages/eslint-plugin/src/rules/no-duplicate-enum-values.ts
@@ -0,0 +1,72 @@
+import { AST_NODE_TYPES, TSESTree } from '@typescript-eslint/utils';
+import * as util from '../util';
+
+export default util.createRule({
+  name: 'no-duplicate-enum-values',
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'Disallow duplicate enum member values',
+      recommended: false,
+    },
+    hasSuggestions: true,
+    messages: {
+      duplicateValue: 'Duplicate enum member value {{value}}.',
+    },
+    schema: [],
+  },
+  defaultOptions: [],
+  create(context) {
+    function isStringLiteral(
+      node: TSESTree.Expression,
+    ): node is TSESTree.StringLiteral {
+      return (
+        node.type === AST_NODE_TYPES.Literal && typeof node.value === 'string'
+      );
+    }
+
+    function isNumberLiteral(
+      node: TSESTree.Expression,
+    ): node is TSESTree.NumberLiteral {
+      return (
+        node.type === AST_NODE_TYPES.Literal && typeof node.value === 'number'
+      );
+    }
+
+    return {
+      TSEnumDeclaration(node: TSESTree.TSEnumDeclaration): void {
+        const enumMembers = node.members;
+        const seenValues = new Set<number | string>();
+
+        enumMembers.forEach(member => {
+          if (member.initializer === undefined) {
+            return;
+          }
+
+          let value: string | number | undefined;
+          if (isStringLiteral(member.initializer)) {
+            value = String(member.initializer.value);
+          } else if (isNumberLiteral(member.initializer)) {
+            value = Number(member.initializer.value);
+          }
+
+          if (value === undefined) {
+            return;
+          }
+
+          if (seenValues.has(value)) {
+            context.report({
+              node: member,
+              messageId: 'duplicateValue',
+              data: {
+                value,
+              },
+            });
+          } else {
+            seenValues.add(value);
+          }
+        });
+      },
+    };
+  },
+});
diff --git a/packages/eslint-plugin/tests/rules/no-duplicate-enum-values.test.ts b/packages/eslint-plugin/tests/rules/no-duplicate-enum-values.test.ts
@@ -0,0 +1,136 @@
+import rule from '../../src/rules/no-duplicate-enum-values';
+import { RuleTester } from '../RuleTester';
+
+const ruleTester = new RuleTester({
+  parser: '@typescript-eslint/parser',
+});
+
+ruleTester.run('no-duplicate-enum-values', rule, {
+  valid: [
+    `
+enum E {
+  A,
+  B,
+}
+    `,
+    `
+enum E {
+  A = 1,
+  B,
+}
+    `,
+    `
+enum E {
+  A = 1,
+  B = 2,
+}
+    `,
+    `
+enum E {
+  A = 'A',
+  B = 'B',
+}
+    `,
+    `
+enum E {
+  A = 'A',
+  B = 'B',
+  C,
+}
+    `,
+    `
+enum E {
+  A = 'A',
+  B = 'B',
+  C = 2,
+  D = 1 + 1,
+}
+    `,
+    `
+enum E {
+  A = 3,
+  B = 2,
+  C,
+}
+    `,
+    `
+enum E {
+  A = 'A',
+  B = 'B',
+  C = 2,
+  D = foo(),
+}
+    `,
+    `
+enum E {
+  A = '',
+  B = 0,
+}
+    `,
+    `
+enum E {
+  A = 0,
+  B = -0,
+  C = NaN,
+}
+    `,
+  ],
+  invalid: [
+    {
+      code: `
+enum E {
+  A = 1,
+  B = 1,
+}
+      `,
+      errors: [
+        {
+          line: 4,
+          column: 3,
+          messageId: 'duplicateValue',
+          data: { value: 1 },
+        },
+      ],
+    },
+    {
+      code: `
+enum E {
+  A = 'A',
+  B = 'A',
+}
+      `,
+      errors: [
+        {
+          line: 4,
+          column: 3,
+          messageId: 'duplicateValue',
+          data: { value: 'A' },
+        },
+      ],
+    },
+    {
+      code: `
+enum E {
+  A = 'A',
+  B = 'A',
+  C = 1,
+  D = 1,
+}
+      `,
+      errors: [
+        {
+          line: 4,
+          column: 3,
+          messageId: 'duplicateValue',
+          data: { value: 'A' },
+        },
+        {
+          line: 6,
+          column: 3,
+          messageId: 'duplicateValue',
+          data: { value: 1 },
+        },
+      ],
+    },
+  ],
+});