typescript-eslint · JoshuaKGoldberg · Oct 17, 2023 · Sep 27, 2023 · Sep 27, 2023 · Sep 27, 2023
diff --git a/packages/eslint-plugin/docs/rules/max-params.md b/packages/eslint-plugin/docs/rules/max-params.md
@@ -0,0 +1,10 @@
+---
+description: 'Enforce a maximum number of parameters in function definitions.'
+---
+
+> 🛑 This file is source code, not the primary documentation location! 🛑
+>
+> See **https://typescript-eslint.io/rules/max-params** for documentation.
+
+This rule extends the base [`eslint/max-params`](https://eslint.org/docs/rules/max-params) rule.
+This version adds support for TypeScript `this` parameters so they won't be counted as a parameter.
diff --git a/packages/eslint-plugin/src/configs/all.ts b/packages/eslint-plugin/src/configs/all.ts
@@ -52,6 +52,8 @@ export = {
    '@typescript-eslint/lines-around-comment': 'error',
    'lines-between-class-members': 'off',
    '@typescript-eslint/lines-between-class-members': 'error',
+    'max-params': 'off',
+    '@typescript-eslint/max-params': 'error',
    '@typescript-eslint/member-delimiter-style': 'error',
    '@typescript-eslint/member-ordering': 'error',
    '@typescript-eslint/method-signature-style': 'error',

diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
@@ -28,6 +28,7 @@ import keySpacing from './key-spacing';
 import keywordSpacing from './keyword-spacing';
 import linesAroundComment from './lines-around-comment';
 import linesBetweenClassMembers from './lines-between-class-members';
+import maxParams from './max-params';
 import memberDelimiterStyle from './member-delimiter-style';
 import memberOrdering from './member-ordering';
 import methodSignatureStyle from './method-signature-style';
@@ -164,6 +165,7 @@ export default {
  'keyword-spacing': keywordSpacing,
  'lines-around-comment': linesAroundComment,
  'lines-between-class-members': linesBetweenClassMembers,
+  'max-params': maxParams,
  'member-delimiter-style': memberDelimiterStyle,
  'member-ordering': memberOrdering,
  'method-signature-style': methodSignatureStyle,

diff --git a/packages/eslint-plugin/src/rules/max-params.ts b/packages/eslint-plugin/src/rules/max-params.ts
@@ -0,0 +1,92 @@
+import { AST_NODE_TYPES, type TSESTree } from '@typescript-eslint/utils';
+
+import type {
+  InferMessageIdsTypeFromRule,
+  InferOptionsTypeFromRule,
+} from '../util';
+import { createRule } from '../util';
+import { getESLintCoreRule } from '../util/getESLintCoreRule';
+
+type FunctionLike =
+  | TSESTree.FunctionDeclaration
+  | TSESTree.FunctionExpression
+  | TSESTree.ArrowFunctionExpression;
+
+type FunctionRuleListener<T extends FunctionLike> = (node: T) => void;
+
+const baseRule = getESLintCoreRule('max-params');
+
+export type Options = InferOptionsTypeFromRule<typeof baseRule>;
+export type MessageIds = InferMessageIdsTypeFromRule<typeof baseRule>;
+
+export default createRule<Options, MessageIds>({
+  name: 'max-params',
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description:
+        'Enforce a maximum number of parameters in function definitions',
+      extendsBaseRule: true,
+    },
+    schema: [
+      {
+        type: 'object',
+        properties: {
+          maximum: {
+            type: 'integer',
+            minimum: 0,
+          },
+          max: {
+            type: 'integer',
+            minimum: 0,
+          },
+          countVoidThis: {
+            type: 'boolean',
+          },
+        },
+        additionalProperties: false,
+      },
+    ],
+    messages: baseRule.meta.messages,
+  },
+  defaultOptions: [{ max: 3, countVoidThis: false }],
+
+  create(context, [{ countVoidThis }]) {
+    const baseRules = baseRule.create(context);
+
+    if (countVoidThis === true) {
+      return baseRules;
+    }
+
+    const removeVoidThisParam = <T extends FunctionLike>(node: T): T => {
+      if (
+        node.params.length === 0 ||
+        node.params[0].type !== AST_NODE_TYPES.Identifier ||
+        node.params[0].name !== 'this' ||
+        node.params[0].typeAnnotation?.typeAnnotation.type !==
+          AST_NODE_TYPES.TSVoidKeyword
+      ) {
+        return node;
+      }
+
+      return {
+        ...node,
+        params: node.params.slice(1),
+      };
+    };
+
+    const wrapListener = <T extends FunctionLike>(
+      listener: FunctionRuleListener<T>,
+    ): FunctionRuleListener<T> => {
+      return (node: T): void => {
+        listener(removeVoidThisParam(node));
+      };
+    };
+
+    return {
+      ArrowFunctionExpression: wrapListener(baseRules.ArrowFunctionExpression),
+      FunctionDeclaration: wrapListener(baseRules.FunctionDeclaration),
+      FunctionExpression: wrapListener(baseRules.FunctionExpression),
+    };
+  },
+});
diff --git a/packages/eslint-plugin/src/util/getESLintCoreRule.ts b/packages/eslint-plugin/src/util/getESLintCoreRule.ts
@@ -17,6 +17,7 @@ interface RuleMap {
  'keyword-spacing': typeof import('eslint/lib/rules/keyword-spacing');
  'lines-around-comment': typeof import('eslint/lib/rules/lines-around-comment');
  'lines-between-class-members': typeof import('eslint/lib/rules/lines-between-class-members');
+  'max-params': typeof import('eslint/lib/rules/max-params');
  'no-dupe-args': typeof import('eslint/lib/rules/no-dupe-args');
  'no-dupe-class-members': typeof import('eslint/lib/rules/no-dupe-class-members');
  'no-empty-function': typeof import('eslint/lib/rules/no-empty-function');

diff --git a/packages/eslint-plugin/tests/rules/max-params.test.ts b/packages/eslint-plugin/tests/rules/max-params.test.ts
@@ -0,0 +1,104 @@
+import { RuleTester } from '@typescript-eslint/rule-tester';
+
+import rule from '../../src/rules/max-params';
+
+const ruleTester = new RuleTester({
+  parser: '@typescript-eslint/parser',
+});
+
+ruleTester.run('max-params', rule, {
+  valid: [
+    'function foo() {}',
+    'const foo = function () {};',
+    'const foo = () => {};',
+    'function foo(a) {}',
+    `
+class Foo {
+  constructor(a) {}
+}
+    `,
+    `
+class Foo {
+  method(this: void, a, b, c) {}
+}
+    `,
+    `
+class Foo {
+  method(this: Foo, a, b) {}
+}
+    `,
+    {
+      code: 'function foo(a, b, c, d) {}',
+      options: [{ max: 4 }],
+    },
+    {
+      code: 'function foo(a, b, c, d) {}',
+      options: [{ maximum: 4 }],
+    },
+    {
+      code: `
+class Foo {
+  method(this: void) {}
+}
+      `,
+      options: [{ max: 0 }],
+    },
+    {
+      code: `
+class Foo {
+  method(this: void, a) {}
+}
+      `,
+      options: [{ max: 1 }],
+    },
+    {
+      code: `
+class Foo {
+  method(this: void, a) {}
+}
+      `,
+      options: [{ max: 2, countVoidThis: true }],
+    },
+  ],
+  invalid: [
+    { code: 'function foo(a, b, c, d) {}', errors: [{ messageId: 'exceed' }] },
+    {
+      code: 'const foo = function (a, b, c, d) {};',
+      errors: [{ messageId: 'exceed' }],
+    },
+    {
+      code: 'const foo = (a, b, c, d) => {};',
+      errors: [{ messageId: 'exceed' }],
+    },
+    {
+      code: 'const foo = a => {};',
+      options: [{ max: 0 }],
+      errors: [{ messageId: 'exceed' }],
+    },
+    {
+      code: `
+class Foo {
+  method(this: void, a, b, c, d) {}
+}
+      `,
+      errors: [{ messageId: 'exceed' }],
+    },
+    {
+      code: `
+class Foo {
+  method(this: void, a) {}
+}
+      `,
+      options: [{ max: 1, countVoidThis: true }],
+      errors: [{ messageId: 'exceed' }],
+    },
+    {
+      code: `
+class Foo {
+  method(this: Foo, a, b, c) {}
+}
+      `,
+      errors: [{ messageId: 'exceed' }],
+    },
+  ],
+});
diff --git a/packages/eslint-plugin/tests/schema-snapshots/max-params.shot b/packages/eslint-plugin/tests/schema-snapshots/max-params.shot
diff --git a/packages/eslint-plugin/typings/eslint-rules.d.ts b/packages/eslint-plugin/typings/eslint-rules.d.ts
@@ -272,6 +272,24 @@ declare module 'eslint/lib/rules/keyword-spacing' {
  export = rule;
 }

+declare module 'eslint/lib/rules/max-params' {
+  import type { TSESLint, TSESTree } from '@typescript-eslint/utils';
+
+  const rule: TSESLint.RuleModule<
+    'exceed',
+    (
+      | { max: number; countVoidThis?: boolean }
+      | { maximum: number; countVoidThis?: boolean }
+    )[],
+    {
+      FunctionDeclaration(node: TSESTree.FunctionDeclaration): void;
+      FunctionExpression(node: TSESTree.FunctionExpression): void;
+      ArrowFunctionExpression(node: TSESTree.ArrowFunctionExpression): void;
+    }
+  >;
+  export = rule;
+}
+
 declare module 'eslint/lib/rules/no-dupe-class-members' {
  import type { TSESLint, TSESTree } from '@typescript-eslint/utils';