From f5cf2339051858492de52b1d3847e6615def0c12 Mon Sep 17 00:00:00 2001
From: Kirk Waiblinger <kirk.waiblinger@gmail.com>
Date: Mon, 16 Sep 2024 02:52:44 -0600
Subject: [PATCH 1/4] ban !in and !instanceof

---
 .../rules/no-confusing-non-null-assertion.mdx |  19 +-
 .../rules/no-confusing-non-null-assertion.ts  | 164 ++++++++++++++----
 .../no-confusing-non-null-assertion.test.ts   |  73 +++++++-
 3 files changed, 218 insertions(+), 38 deletions(-)

diff --git a/packages/eslint-plugin/docs/rules/no-confusing-non-null-assertion.mdx b/packages/eslint-plugin/docs/rules/no-confusing-non-null-assertion.mdx
index 0ff48e14c926..01a47a9dde78 100644
--- a/packages/eslint-plugin/docs/rules/no-confusing-non-null-assertion.mdx
+++ b/packages/eslint-plugin/docs/rules/no-confusing-non-null-assertion.mdx
@@ -9,14 +9,27 @@ import TabItem from '@theme/TabItem';
 >
 > See **https://typescript-eslint.io/rules/no-confusing-non-null-assertion** for documentation.
 
-Using a non-null assertion (`!`) next to an assign or equals check (`=` or `==` or `===`) creates code that is confusing as it looks similar to a not equals check (`!=` `!==`).
+Using a non-null assertion (`!`) next to an assignment or equality check (`=` or `==` or `===`) creates code that is confusing as it looks similar to an inequality check (`!=` `!==`).
 
 ```typescript
-a! == b; // a non-null assertions(`!`) and an equals test(`==`)
+a! == b; // a non-null assertion(`!`) and an equals test(`==`)
 a !== b; // not equals test(`!==`)
-a! === b; // a non-null assertions(`!`) and an triple equals test(`===`)
+a! === b; // a non-null assertion(`!`) and a triple equals test(`===`)
 ```
 
+Using a non-null assertion (`!`) next to an in test (`in`) or an instanceof test (`instanceof`) creates code that is confusing since it may look like the operator is negated, but it is actually not.
+
+{/* prettier-ignore */}
+```typescript
+a! in b; // a non-null assertion(`!`) and an in test(`in`)
+a !in b; // also a non-null assertion(`!`) and an in test(`in`)
+!(a in b); // a negated in test
+
+a! instanceof b; // a non-null assertion(`!`) and an instanceof test(`instanceof`)
+a !instanceof b; // also a non-null assertion(`!`) and an instanceof test(`instanceof`)
+!(a instanceof b); // a negated instanceof test
+````
+
 This rule flags confusing `!` assertions and suggests either removing them or wrapping the asserted expression in `()` parenthesis.
 
 ## Examples
diff --git a/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts b/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
index 9caf777aabad..83a6a03fea06 100644
--- a/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
+++ b/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
@@ -1,9 +1,39 @@
+import { MessagingOptions } from 'node:child_process';
+
 import type { TSESLint, TSESTree } from '@typescript-eslint/utils';
 import { AST_NODE_TYPES, AST_TOKEN_TYPES } from '@typescript-eslint/utils';
+import type {
+  ReportDescriptor,
+  RuleFix,
+  SuggestionReportDescriptor,
+} from '@typescript-eslint/utils/ts-eslint';
 
 import { createRule } from '../util';
 
-export default createRule({
+type MessageId =
+  | 'confusingEqual'
+  | 'confusingAssign'
+  | 'confusingOperator'
+  | 'notNeedInEqualTest'
+  | 'notNeedInAssign'
+  | 'notNeedInOperator'
+  | 'wrapUpLeft';
+
+const confusingOperators = new Set([
+  '=',
+  '==',
+  '===',
+  'in',
+  'instanceof',
+] as const);
+type ConfusingOperator =
+  typeof confusingOperators extends Set<infer T> ? T : never;
+
+function isConfusingOperator(operator: string): operator is ConfusingOperator {
+  return confusingOperators.has(operator as ConfusingOperator);
+}
+
+export default createRule<[], MessageId>({
   name: 'no-confusing-non-null-assertion',
   meta: {
     type: 'problem',
@@ -15,35 +45,62 @@ export default createRule({
     hasSuggestions: true,
     messages: {
       confusingEqual:
-        'Confusing combinations of non-null assertion and equal test like "a! == b", which looks very similar to not equal "a !== b".',
+        'Confusing combination of non-null assertion and equality test like `a! == b`, which looks very similar to `a !== b`.',
       confusingAssign:
-        'Confusing combinations of non-null assertion and equal test like "a! = b", which looks very similar to not equal "a != b".',
-      notNeedInEqualTest: 'Unnecessary non-null assertion (!) in equal test.',
+        'Confusing combination of non-null assertion and assignment like `a! = b`, which looks very similar to `a != b`.',
+      confusingOperator:
+        'Confusing combination of non-null assertion and `{{operator}}` operator like `a! {{operator}} b`, which might be misinterpreted as `!(a {{operator}} b)`.',
+
+      notNeedInEqualTest:
+        'Remove unnecessary non-null assertion (!) in equality test.',
       notNeedInAssign:
-        'Unnecessary non-null assertion (!) in assignment left hand.',
+        'Remove unnecessary non-null assertion (!) in assignment left-hand side.',
+
+      notNeedInOperator:
+        'Remove possibly unnecessary non-null assertion (!) in the left operand of the `{{operator}}` operator.',
+
       wrapUpLeft:
-        'Wrap up left hand to avoid putting non-null assertion "!" and "=" together.',
+        'Wrap the left-hand side in parentheses to avoid confusion with "{{operator}}" operator.',
     },
     schema: [],
   },
   defaultOptions: [],
   create(context) {
+    function confusingOperatorToMessageData(
+      operator: ConfusingOperator,
+    ): Pick<ReportDescriptor<MessageId>, 'messageId' | 'data'> {
+      switch (operator) {
+        case '=':
+          return {
+            messageId: 'confusingAssign',
+          };
+        case '==':
+        case '===':
+          return {
+            messageId: 'confusingEqual',
+          };
+        case 'in':
+        case 'instanceof':
+          return {
+            messageId: 'confusingOperator',
+            data: { operator },
+          };
+        default:
+          operator satisfies never;
+          throw new Error(`Unexpected operator ${operator as string}`);
+      }
+    }
+
     return {
       'BinaryExpression, AssignmentExpression'(
         node: TSESTree.AssignmentExpression | TSESTree.BinaryExpression,
       ): void {
-        function isLeftHandPrimaryExpression(
-          node: TSESTree.Expression | TSESTree.PrivateIdentifier,
-        ): boolean {
-          return node.type === AST_NODE_TYPES.TSNonNullExpression;
-        }
+        const operator = node.operator;
 
-        if (
-          node.operator === '==' ||
-          node.operator === '===' ||
-          node.operator === '='
-        ) {
-          const isAssign = node.operator === '=';
+        if (isConfusingOperator(operator)) {
+          // Look for a non-null assertion as the last token on the left hand side.
+          // That way, we catch things like `1 + two! === 3`, even though the left
+          // hand side isn't a non-null assertion AST node.
           const leftHandFinalToken = context.sourceCode.getLastToken(node.left);
           const tokenAfterLeft = context.sourceCode.getTokenAfter(node.left);
           if (
@@ -51,32 +108,62 @@ export default createRule({
             leftHandFinalToken.value === '!' &&
             tokenAfterLeft?.value !== ')'
           ) {
-            if (isLeftHandPrimaryExpression(node.left)) {
+            if (node.left.type === AST_NODE_TYPES.TSNonNullExpression) {
+              let suggestions: TSESLint.SuggestionReportDescriptor<MessageId>[];
+              switch (operator) {
+                case '=':
+                  suggestions = [
+                    {
+                      messageId: 'notNeedInAssign',
+                      fix: (fixer): RuleFix => fixer.remove(leftHandFinalToken),
+                    },
+                  ];
+                  break;
+
+                case '==':
+                case '===':
+                  suggestions = [
+                    {
+                      messageId: 'notNeedInEqualTest',
+                      fix: (fixer): RuleFix => fixer.remove(leftHandFinalToken),
+                    },
+                  ];
+                  break;
+
+                case 'in':
+                case 'instanceof':
+                  suggestions = [
+                    {
+                      messageId: 'notNeedInOperator',
+                      data: { operator },
+                      fix: (fixer): RuleFix => fixer.remove(leftHandFinalToken),
+                    },
+                    {
+                      messageId: 'wrapUpLeft',
+                      data: { operator },
+                      fix: wrapUpLeftFixer(node),
+                    },
+                  ];
+                  break;
+
+                default:
+                  operator satisfies never;
+                  return;
+              }
               context.report({
                 node,
-                messageId: isAssign ? 'confusingAssign' : 'confusingEqual',
-                suggest: [
-                  {
-                    messageId: isAssign
-                      ? 'notNeedInAssign'
-                      : 'notNeedInEqualTest',
-                    fix: (fixer): TSESLint.RuleFix[] => [
-                      fixer.remove(leftHandFinalToken),
-                    ],
-                  },
-                ],
+                ...confusingOperatorToMessageData(operator),
+                suggest: suggestions,
               });
             } else {
               context.report({
                 node,
-                messageId: isAssign ? 'confusingAssign' : 'confusingEqual',
+                ...confusingOperatorToMessageData(operator),
                 suggest: [
                   {
                     messageId: 'wrapUpLeft',
-                    fix: (fixer): TSESLint.RuleFix[] => [
-                      fixer.insertTextBefore(node.left, '('),
-                      fixer.insertTextAfter(node.left, ')'),
-                    ],
+                    data: { operator },
+                    fix: wrapUpLeftFixer(node),
                   },
                 ],
               });
@@ -87,3 +174,12 @@ export default createRule({
     };
   },
 });
+
+function wrapUpLeftFixer(
+  node: TSESTree.AssignmentExpression | TSESTree.BinaryExpression,
+): TSESLint.ReportFixFunction {
+  return (fixer): TSESLint.RuleFix[] => [
+    fixer.insertTextBefore(node.left, '('),
+    fixer.insertTextAfter(node.left, ')'),
+  ];
+}
diff --git a/packages/eslint-plugin/tests/rules/no-confusing-non-null-assertion.test.ts b/packages/eslint-plugin/tests/rules/no-confusing-non-null-assertion.test.ts
index 19f3cfec601d..6cdefc9b6c59 100644
--- a/packages/eslint-plugin/tests/rules/no-confusing-non-null-assertion.test.ts
+++ b/packages/eslint-plugin/tests/rules/no-confusing-non-null-assertion.test.ts
@@ -3,7 +3,7 @@
 /* eslint "@typescript-eslint/internal/plugin-test-formatting": ["error", { formatWithPrettier: false }] */
 /* eslint-enable eslint-comments/no-use */
 
-import { RuleTester } from '@typescript-eslint/rule-tester';
+import { noFormat, RuleTester } from '@typescript-eslint/rule-tester';
 
 import rule from '../../src/rules/no-confusing-non-null-assertion';
 
@@ -18,6 +18,8 @@ ruleTester.run('no-confusing-non-null-assertion', rule, {
     'a != b;',
     '(a + b!) == c;',
     '(a + b!) = c;',
+    '(a + b!) in c;',
+    '(a || b!) instanceof c;',
   ],
   invalid: [
     {
@@ -148,5 +150,74 @@ ruleTester.run('no-confusing-non-null-assertion', rule, {
         },
       ],
     },
+    {
+      code: 'a! in b;',
+      errors: [
+        {
+          messageId: 'confusingOperator',
+          data: { operator: 'in' },
+          line: 1,
+          column: 1,
+          suggestions: [
+            {
+              messageId: 'notNeedInOperator',
+              output: 'a in b;',
+            },
+            {
+              messageId: 'wrapUpLeft',
+              output: '(a!) in b;',
+            },
+          ],
+        },
+      ],
+    },
+    {
+      code: noFormat`
+a !in b;
+      `,
+      errors: [
+        {
+          messageId: 'confusingOperator',
+          data: { operator: 'in' },
+          line: 2,
+          column: 1,
+          suggestions: [
+            {
+              messageId: 'notNeedInOperator',
+              output: `
+a in b;
+      `,
+            },
+            {
+              messageId: 'wrapUpLeft',
+              output: `
+(a !)in b;
+      `,
+            },
+          ],
+        },
+      ],
+    },
+    {
+      code: 'a! instanceof b;',
+      errors: [
+        {
+          messageId: 'confusingOperator',
+          data: { operator: 'instanceof' },
+          line: 1,
+          column: 1,
+          suggestions: [
+            {
+              messageId: 'notNeedInOperator',
+              output: 'a instanceof b;',
+            },
+            {
+              messageId: 'wrapUpLeft',
+              output: '(a!) instanceof b;',
+            },
+          ],
+        },
+      ],
+    },
   ],
 });

From 06527974bec3a3c33b7c0ca7ae3d91a046ea4bdb Mon Sep 17 00:00:00 2001
From: Kirk Waiblinger <kirk.waiblinger@gmail.com>
Date: Mon, 16 Sep 2024 03:03:09 -0600
Subject: [PATCH 2/4] lintfix

---
 .../eslint-plugin/src/rules/no-confusing-non-null-assertion.ts | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts b/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
index 83a6a03fea06..3185e1e54be6 100644
--- a/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
+++ b/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
@@ -1,11 +1,8 @@
-import { MessagingOptions } from 'node:child_process';
-
 import type { TSESLint, TSESTree } from '@typescript-eslint/utils';
 import { AST_NODE_TYPES, AST_TOKEN_TYPES } from '@typescript-eslint/utils';
 import type {
   ReportDescriptor,
   RuleFix,
-  SuggestionReportDescriptor,
 } from '@typescript-eslint/utils/ts-eslint';
 
 import { createRule } from '../util';

From 76298e152e75923714b43e4821746cdc9c872c3e Mon Sep 17 00:00:00 2001
From: Kirk Waiblinger <kirk.waiblinger@gmail.com>
Date: Sun, 22 Sep 2024 19:47:15 -0600
Subject: [PATCH 3/4] snapshots

---
 .../no-confusing-non-null-assertion.shot                      | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/eslint-plugin/tests/docs-eslint-output-snapshots/no-confusing-non-null-assertion.shot b/packages/eslint-plugin/tests/docs-eslint-output-snapshots/no-confusing-non-null-assertion.shot
index 074f8f721db5..fc3fe7bcc1be 100644
--- a/packages/eslint-plugin/tests/docs-eslint-output-snapshots/no-confusing-non-null-assertion.shot
+++ b/packages/eslint-plugin/tests/docs-eslint-output-snapshots/no-confusing-non-null-assertion.shot
@@ -10,9 +10,9 @@ interface Foo {
 
 const foo: Foo = getFoo();
 const isEqualsBar = foo.bar! == 'hello';
-                    ~~~~~~~~~~~~~~~~~~~ Confusing combinations of non-null assertion and equal test like "a! == b", which looks very similar to not equal "a !== b".
+                    ~~~~~~~~~~~~~~~~~~~ Confusing combination of non-null assertion and equality test like \`a! == b\`, which looks very similar to \`a !== b\`.
 const isEqualsNum = 1 + foo.num! == 2;
-                    ~~~~~~~~~~~~~~~~~ Confusing combinations of non-null assertion and equal test like "a! == b", which looks very similar to not equal "a !== b".
+                    ~~~~~~~~~~~~~~~~~ Confusing combination of non-null assertion and equality test like \`a! == b\`, which looks very similar to \`a !== b\`.
 "
 `;
 

From e3172b7b82d3e7cb2b434518a579a9767d16e52f Mon Sep 17 00:00:00 2001
From: Kirk Waiblinger <kirk.waiblinger@gmail.com>
Date: Sun, 22 Sep 2024 20:17:49 -0600
Subject: [PATCH 4/4] cov

---
 .../eslint-plugin/src/rules/no-confusing-non-null-assertion.ts  | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts b/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
index 3185e1e54be6..160827fc5a32 100644
--- a/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
+++ b/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
@@ -82,6 +82,7 @@ export default createRule<[], MessageId>({
             messageId: 'confusingOperator',
             data: { operator },
           };
+        // istanbul ignore next
         default:
           operator satisfies never;
           throw new Error(`Unexpected operator ${operator as string}`);
@@ -143,6 +144,7 @@ export default createRule<[], MessageId>({
                   ];
                   break;
 
+                // istanbul ignore next
                 default:
                   operator satisfies never;
                   return;

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