Extract rule: template-no-chained-this

Author: NullVoxPopuli

README.md

@@ -195,6 +195,7 @@ rules in templates can be disabled with eslint directives with mustache or html
 | [template-no-action-modifiers](docs/rules/template-no-action-modifiers.md)                       | disallow usage of {{action}} modifiers                    |    |    |    |
 | [template-no-arguments-for-html-elements](docs/rules/template-no-arguments-for-html-elements.md) | disallow @arguments on HTML elements                      |    |    |    |
 | [template-no-array-prototype-extensions](docs/rules/template-no-array-prototype-extensions.md)   | disallow usage of Ember Array prototype extensions        |    |    |    |
+| [template-no-chained-this](docs/rules/template-no-chained-this.md)                               | disallow redundant `this.this` in templates               |    | 🔧 |    |
 | [template-no-debugger](docs/rules/template-no-debugger.md)                                       | disallow {{debugger}} in templates                        |    |    |    |
 | [template-no-log](docs/rules/template-no-log.md)                                                 | disallow {{log}} in templates                             |    |    |    |
 

docs/rules/template-no-chained-this.md

@@ -0,0 +1,64 @@
+# ember/template-no-chained-this
+
+🔧 This rule is automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/latest/user-guide/command-line-interface#--fix).
+
+<!-- end auto-generated rule header -->
+
+Disallow chained property access on `this`.
+
+Accessing deeply nested properties through `this` (like `this.user.name`) in templates makes components harder to refactor and test. It also creates tight coupling between the template and the component's internal structure. Use local variables or computed properties instead.
+
+## Rule Details
+
+This rule disallows chaining property access on `this` in templates (e.g., `this.foo.bar`).
+
+## Examples
+
+### Incorrect ❌
+
+```gjs
+<template>
+  {{this.user.name}}
+</template>
+```
+
+```gjs
+<template>
+  {{this.model.user.firstName}}
+</template>
+```
+
+```gjs
+<template>
+  <div>{{this.data.items.length}}</div>
+</template>
+```
+
+### Correct ✅
+
+```gjs
+<template>
+  {{this.userName}}
+</template>
+```
+
+```gjs
+<template>
+  {{get this.user "name"}}
+</template>
+```
+
+```gjs
+<template>
+  {{userName}}
+</template>
+```
+
+## Related Rules
+
+- [template-no-implicit-this](./template-no-implicit-this.md)
+
+## References
+
+- [Ember Best Practices - Component Design](https://guides.emberjs.com/release/components/)
+- [eslint-plugin-ember template-no-this-in-template-only-components](https://github.com/ember-cli/eslint-plugin-ember/blob/master/docs/rules/template-no-this-in-template-only-components.md)

lib/rules/template-no-chained-this.js

@@ -0,0 +1,46 @@
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'disallow redundant `this.this` in templates',
+      category: 'Best Practices',
+      strictGjs: true,
+      strictGts: true,
+      url: 'https://github.com/ember-cli/eslint-plugin-ember/tree/master/docs/rules/template-no-chained-this.md',
+    },
+    fixable: 'code',
+    schema: [],
+    messages: {
+      noChainedThis:
+        'this.this.* is not allowed in templates. This is likely a mistake — remove the redundant this.',
+    },
+  },
+
+  create(context) {
+    const sourceCode = context.sourceCode;
+
+    return {
+      GlimmerPathExpression(node) {
+        if (node.original?.startsWith('this.this.')) {
+          context.report({
+            node,
+            messageId: 'noChainedThis',
+            fix(fixer) {
+              const text = sourceCode.getText(node);
+              return fixer.replaceText(node, text.replace('this.this.', 'this.'));
+            },
+          });
+        }
+      },
+      GlimmerElementNode(node) {
+        if (node.tag?.startsWith('this.this.')) {
+          context.report({
+            node,
+            messageId: 'noChainedThis',
+          });
+        }
+      },
+    };
+  },
+};

tests/lib/rules/template-no-chained-this.js

@@ -0,0 +1,41 @@
+const rule = require('../../../lib/rules/template-no-chained-this');
+const RuleTester = require('eslint').RuleTester;
+
+const ruleTester = new RuleTester({
+  parser: require.resolve('ember-eslint-parser'),
+  parserOptions: { ecmaVersion: 2022, sourceType: 'module' },
+});
+
+ruleTester.run('template-no-chained-this', rule, {
+  valid: [
+    '<template>{{this.value}}</template>',
+    '<template>{{this.thisvalue}}</template>',
+    '<template>{{@argName}}</template>',
+    '<template>{{this.user.name}}</template>',
+    '<template><this.Component /></template>',
+    '<template>{{component this.dynamicComponent}}</template>',
+  ],
+
+  invalid: [
+    {
+      code: '<template>{{this.this.value}}</template>',
+      output: '<template>{{this.value}}</template>',
+      errors: [{ messageId: 'noChainedThis' }],
+    },
+    {
+      code: '<template>{{helper value=this.this.foo}}</template>',
+      output: '<template>{{helper value=this.foo}}</template>',
+      errors: [{ messageId: 'noChainedThis' }],
+    },
+    {
+      code: '<template>{{#if this.this.condition}}true{{/if}}</template>',
+      output: '<template>{{#if this.condition}}true{{/if}}</template>',
+      errors: [{ messageId: 'noChainedThis' }],
+    },
+    {
+      code: '<template><this.this.Component /></template>',
+      output: null,
+      errors: [{ messageId: 'noChainedThis' }],
+    },
+  ],
+});