Extract rule: template-no-unavailable-this

Author: NullVoxPopuli

README.md

@@ -256,6 +256,7 @@ rules in templates can be disabled with eslint directives with mustache or html
 | [template-no-obsolete-elements](docs/rules/template-no-obsolete-elements.md)                                       | disallow obsolete HTML elements                                              |    |    |    |
 | [template-no-outlet-outside-routes](docs/rules/template-no-outlet-outside-routes.md)                               | disallow {{outlet}} outside of route templates                               |    |    |    |
 | [template-no-page-title-component](docs/rules/template-no-page-title-component.md)                                 | disallow usage of ember-page-title component                                 |    |    |    |
+| [template-no-unavailable-this](docs/rules/template-no-unavailable-this.md)                                         | disallow `this` in templates that are not inside a class or function         |    |    |    |
 | [template-no-unnecessary-component-helper](docs/rules/template-no-unnecessary-component-helper.md)                 | disallow unnecessary component helper                                        |    | 🔧 |    |
 | [template-no-unnecessary-concat](docs/rules/template-no-unnecessary-concat.md)                                     | disallow unnecessary string concatenation                                    |    | 🔧 |    |
 | [template-no-unnecessary-curly-parens](docs/rules/template-no-unnecessary-curly-parens.md)                         | disallow unnecessary parentheses enclosing statements in curlies             |    | 🔧 |    |

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

@@ -0,0 +1,51 @@
+# ember/template-no-unavailable-this
+
+<!-- end auto-generated rule header -->
+
+Disallow `this` in templates that are not inside a class or function.
+
+## Rule Details
+
+In Ember and Glimmer, `this` refers to the component instance. When a `<template>` tag is used at module level (not inside a class body or function), `this` has no meaningful value and will be `undefined`. This rule catches accidental usage of `this` in such templates.
+
+## Examples
+
+Examples of **incorrect** code for this rule:
+
+```gjs
+<template>
+  {{this.name}}
+</template>
+```
+
+```gjs
+<template>
+  {{yield this}}
+</template>
+```
+
+Examples of **correct** code for this rule:
+
+```gjs
+import Component from '@glimmer/component';
+
+class MyComponent extends Component {
+  <template>{{this.name}}</template>
+}
+```
+
+```gjs
+function myComponent() {
+  return <template>{{this.name}}</template>;
+}
+```
+
+```gjs
+<template>
+  {{@value}}
+</template>
+```
+
+## References
+
+- [Glimmer Components](https://guides.emberjs.com/release/upgrading/current-edition/glimmer-components/)

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

@@ -0,0 +1,48 @@
+function isInsideClassOrFunction(node) {
+  let current = node.parent;
+  while (current) {
+    if (
+      current.type === 'ClassBody' ||
+      current.type === 'FunctionExpression' ||
+      current.type === 'FunctionDeclaration'
+    ) {
+      return true;
+    }
+    current = current.parent;
+  }
+  return false;
+}
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'disallow `this` in templates that are not inside a class or function',
+      category: 'Best Practices',
+      url: 'https://github.com/ember-cli/eslint-plugin-ember/tree/master/docs/rules/template-no-unavailable-this.md',
+      templateMode: 'strict',
+    },
+    schema: [],
+    messages: {
+      noUnavailableThis:
+        '`this` is not available in a template that is not inside a class or function.',
+    },
+  },
+
+  create(context) {
+    return {
+      GlimmerPathExpression(node) {
+        if (
+          (node.original === 'this' || node.original?.startsWith('this.')) &&
+          !isInsideClassOrFunction(node)
+        ) {
+          context.report({
+            node,
+            messageId: 'noUnavailableThis',
+          });
+        }
+      },
+    };
+  },
+};

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

@@ -0,0 +1,76 @@
+const rule = require('../../../lib/rules/template-no-unavailable-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-unavailable-this', rule, {
+  valid: [
+    // this inside a class body
+    `import Component from '@glimmer/component';
+     class MyComponent extends Component {
+       <template>{{this.name}}</template>
+     }`,
+    // this inside a function
+    `function myComponent() {
+       return <template>{{this.name}}</template>;
+     }`,
+    // this inside an arrow function that is inside a class (arrow inherits this from class)
+    `import Component from '@glimmer/component';
+     class MyComponent extends Component {
+       foo = () => {
+         return <template>{{this.name}}</template>;
+       }
+     }`,
+    // No this at all
+    '<template>{{@value}}</template>',
+    '<template><div>Content</div></template>',
+    // yield this inside a class (this is valid, other rules handle yield specifics)
+    `import Component from '@glimmer/component';
+     class MyComponent extends Component {
+       <template>{{yield this}}</template>
+     }`,
+  ],
+  invalid: [
+    // this.property at module level
+    {
+      code: '<template>{{this.name}}</template>',
+      output: null,
+      errors: [{ messageId: 'noUnavailableThis' }],
+    },
+    // bare this at module level
+    {
+      code: '<template>{{this}}</template>',
+      output: null,
+      errors: [{ messageId: 'noUnavailableThis' }],
+    },
+    // yield this at module level (this rule catches the `this`, yield rule catches yield semantics)
+    {
+      code: '<template>{{yield this}}</template>',
+      output: null,
+      errors: [{ messageId: 'noUnavailableThis' }],
+    },
+    // multiple this references at module level
+    {
+      code: '<template>{{this.foo}} {{this.bar}}</template>',
+      output: null,
+      errors: [{ messageId: 'noUnavailableThis' }, { messageId: 'noUnavailableThis' }],
+    },
+    // deeply nested this.property at module level
+    {
+      code: '<template>{{this.foo.bar.baz}}</template>',
+      output: null,
+      errors: [{ messageId: 'noUnavailableThis' }],
+    },
+    // arrow function at module level (arrow functions don't have their own this)
+    {
+      code: `const myComponent = () => {
+       return <template>{{this}}</template>;
+     }`,
+      output: null,
+      errors: [{ messageId: 'noUnavailableThis' }],
+    },
+  ],
+});