feat(eslint-plugin): add prefer-enum-initializers rule (#2326)

packages/eslint-plugin/README.md

@@ -147,6 +147,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
 | [`@typescript-eslint/no-unused-vars-experimental`](./docs/rules/no-unused-vars-experimental.md)                       | Disallow unused variables and arguments                                                                                 |                    |          |                   |
 | [`@typescript-eslint/no-var-requires`](./docs/rules/no-var-requires.md)                                               | Disallows the use of require statements except in import statements                                                     | :heavy_check_mark: |          |                   |
 | [`@typescript-eslint/prefer-as-const`](./docs/rules/prefer-as-const.md)                                               | Prefer usage of `as const` over literal type                                                                            | :heavy_check_mark: | :wrench: |                   |
+| [`@typescript-eslint/prefer-enum-initializers`](./docs/rules/prefer-enum-initializers.md)                             | Prefer initializing each enums member value                                                                             |                    |          |                   |
 | [`@typescript-eslint/prefer-for-of`](./docs/rules/prefer-for-of.md)                                                   | Prefer a ‘for-of’ loop over a standard ‘for’ loop if the index is only used to access the array being iterated          |                    |          |                   |
 | [`@typescript-eslint/prefer-function-type`](./docs/rules/prefer-function-type.md)                                     | Use function types instead of interfaces with call signatures                                                           |                    | :wrench: |                   |
 | [`@typescript-eslint/prefer-includes`](./docs/rules/prefer-includes.md)                                               | Enforce `includes` method over `indexOf` method                                                                         |                    | :wrench: | :thought_balloon: |

packages/eslint-plugin/docs/rules/prefer-enum-initializers.md

@@ -0,0 +1,70 @@
+# Prefer initializing each enums member value (`prefer-enum-initializers`)
+
+This rule recommends having each `enum`s member value explicitly initialized.
+
+`enum`s are a practical way to organize semantically related constant values. However, by implicitly defining values, `enum`s can lead to unexpected bugs if it's modified without paying attention to the order of its items.
+
+## Rule Details
+
+`enum`s infers sequential numbers automatically when initializers are omitted:
+
+```ts
+enum Status {
+  Open, // infer 0
+  Closed, // infer 1
+}
+```
+
+If a new member is added to the top of `Status`, both `Open` and `Closed` would have its values altered:
+
+```ts
+enum Status {
+  Pending, // infer 0
+  Open, // infer 1
+  Closed, // infer 2
+}
+```
+
+Examples of **incorrect** code for this rule:
+
+```ts
+enum Status {
+  Open = 1,
+  Close,
+}
+
+enum Direction {
+  Up,
+  Down,
+}
+
+enum Color {
+  Red,
+  Green = 'Green'
+  Blue = 'Blue',
+}
+```
+
+Examples of **correct** code for this rule:
+
+```ts
+enum Status {
+  Open = 'Open',
+  Close = 'Close',
+}
+
+enum Direction {
+  Up = 1,
+  Down = 2,
+}
+
+enum Color {
+  Red = 'Red',
+  Green = 'Green',
+  Blue = 'Blue',
+}
+```
+
+## When Not To Use It
+
+If you don't care about `enum`s having implicit values you can safely disable this rule.

packages/eslint-plugin/src/configs/all.ts

@@ -97,6 +97,7 @@ export = {
     '@typescript-eslint/no-useless-constructor': 'error',
     '@typescript-eslint/no-var-requires': 'error',
     '@typescript-eslint/prefer-as-const': 'error',
+    '@typescript-eslint/prefer-enum-initializers': 'error',
     '@typescript-eslint/prefer-for-of': 'error',
     '@typescript-eslint/prefer-function-type': 'error',
     '@typescript-eslint/prefer-includes': 'error',

packages/eslint-plugin/src/rules/index.ts

@@ -70,6 +70,7 @@ import noUseBeforeDefine from './no-use-before-define';
 import noUselessConstructor from './no-useless-constructor';
 import noVarRequires from './no-var-requires';
 import preferAsConst from './prefer-as-const';
+import preferEnumInitializers from './prefer-enum-initializers';
 import preferForOf from './prefer-for-of';
 import preferFunctionType from './prefer-function-type';
 import preferIncludes from './prefer-includes';
@@ -169,6 +170,7 @@ export default {
   'no-useless-constructor': noUselessConstructor,
   'no-var-requires': noVarRequires,
   'prefer-as-const': preferAsConst,
+  'prefer-enum-initializers': preferEnumInitializers,
   'prefer-for-of': preferForOf,
   'prefer-function-type': preferFunctionType,
   'prefer-includes': preferIncludes,

packages/eslint-plugin/src/rules/prefer-enum-initializers.ts

@@ -0,0 +1,73 @@
+import { TSESTree } from '@typescript-eslint/experimental-utils';
+import * as util from '../util';
+import { TSESLint } from '@typescript-eslint/experimental-utils';
+
+type MessageIds = 'defineInitializer' | 'defineInitializerSuggestion';
+
+export default util.createRule<[], MessageIds>({
+  name: 'prefer-enum-initializers',
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description: 'Prefer initializing each enums member value',
+      category: 'Best Practices',
+      recommended: false,
+      suggestion: true,
+    },
+    messages: {
+      defineInitializer:
+        "The value of the member '{{ name }}' should be explicitly defined",
+      defineInitializerSuggestion:
+        'Can be fixed to {{ name }} = {{ suggested }}',
+    },
+    schema: [],
+  },
+  defaultOptions: [],
+  create(context) {
+    const sourceCode = context.getSourceCode();
+
+    function TSEnumDeclaration(node: TSESTree.TSEnumDeclaration): void {
+      const { members } = node;
+
+      members.forEach((member, index) => {
+        if (member.initializer == null) {
+          const name = sourceCode.getText(member);
+          context.report({
+            node: member,
+            messageId: 'defineInitializer',
+            data: {
+              name,
+            },
+            suggest: [
+              {
+                messageId: 'defineInitializerSuggestion',
+                data: { name, suggested: index },
+                fix: (fixer): TSESLint.RuleFix => {
+                  return fixer.replaceText(member, `${name} = ${index}`);
+                },
+              },
+              {
+                messageId: 'defineInitializerSuggestion',
+                data: { name, suggested: index + 1 },
+                fix: (fixer): TSESLint.RuleFix => {
+                  return fixer.replaceText(member, `${name} = ${index + 1}`);
+                },
+              },
+              {
+                messageId: 'defineInitializerSuggestion',
+                data: { name, suggested: `'${name}'` },
+                fix: (fixer): TSESLint.RuleFix => {
+                  return fixer.replaceText(member, `${name} = '${name}'`);
+                },
+              },
+            ],
+          });
+        }
+      });
+    }
+
+    return {
+      TSEnumDeclaration,
+    };
+  },
+});