Repository: ember-cli/eslint-plugin-ember (js)

Issue:
Propose new rule: no-tracked-properties-from-args
Disallow the creation of `@tracked value = this.args.someValue` as this will not get properly updated when the args change, which is unlikely to be what the developer wants.

Pull request:
Add new rule `no-tracked-properties-from-args`
Fixes #1701 

Patch:
diff --git a/README.md b/README.md
index b5aa064b..536503d1 100644
--- a/README.md
+++ b/README.md
@@ -137,6 +137,7 @@ module.exports = {
 | [no-classic-classes](docs/rules/no-classic-classes.md)                                     | disallow "classic" classes in favor of native JS classes                                                       | ✅  |    |    |
 | [no-ember-super-in-es-classes](docs/rules/no-ember-super-in-es-classes.md)                 | disallow use of `this._super` in ES class methods                                                              | ✅  | 🔧 |    |
 | [no-empty-glimmer-component-classes](docs/rules/no-empty-glimmer-component-classes.md)     | disallow empty backing classes for Glimmer components                                                          | ✅  |    |    |
+| [no-tracked-properties-from-args](docs/rules/no-tracked-properties-from-args.md)           | disallow creating @tracked properties from this.args                                                           |    |    |    |
 
 ### jQuery
 
diff --git a/docs/rules/no-tracked-properties-from-args.md b/docs/rules/no-tracked-properties-from-args.md
new file mode 100644
index 00000000..975233c2
--- /dev/null
+++ b/docs/rules/no-tracked-properties-from-args.md
@@ -0,0 +1,35 @@
+# ember/no-tracked-properties-from-args
+
+<!-- end auto-generated rule header -->
+
+Disallow creation of @tracked properties from args.
+
+## Rule Details
+
+This rule disallows the creation of @tracked properties with values from `this.args`. The @tracked property will not be updated when the args change, which is almost never what you want. Instead, use a getter to derive the desired state.
+If you need to modify a specific arg, consider having the parent provide a way for the child component to update it. This avoids having two sources of truth that you will need to keep in sync.
+
+## Examples
+
+Examples of **incorrect** code for this rule:
+
+```js
+class Example {
+    @tracked someValue = this.args.someValue;
+}
+```
+
+Examples of **correct** code for this rule:
+
+```js
+class Example {
+    get someValue() {
+        return this.args.someValue;
+    }
+}
+```
+
+## References
+
+- [Glimmer Components - args](https://guides.emberjs.com/release/upgrading/current-edition/glimmer-components/#toc_getting-used-to-glimmer-components) guide
+- [tracked](https://guides.emberjs.com/release/in-depth-topics/autotracking-in-depth/) guide
diff --git a/lib/rules/no-tracked-properties-from-args.js b/lib/rules/no-tracked-properties-from-args.js
new file mode 100644
index 00000000..86354f40
--- /dev/null
+++ b/lib/rules/no-tracked-properties-from-args.js
@@ -0,0 +1,62 @@
+'use strict';
+
+const { getImportIdentifier } = require('../utils/import');
+const { isClassPropertyOrPropertyDefinitionWithDecorator } = require('../utils/decorators');
+
+const ERROR_MESSAGE =
+  'Do not use this.args to create @tracked properties as this will not be updated if the args change. Instead use a getter.';
+
+function visitClassPropertyOrPropertyDefinition(node, context, trackedImportName) {
+  const hasTrackedDecorator =
+    node.decorators?.length > 0 &&
+    isClassPropertyOrPropertyDefinitionWithDecorator(node, trackedImportName);
+
+  const hasThisArgsValue =
+    node.value.object?.type === 'MemberExpression' &&
+    node.value.object?.object.type === 'ThisExpression' &&
+    node.value.object?.property.name === 'args';
+
+  if (hasTrackedDecorator && hasThisArgsValue) {
+    context.report({ node, messageId: 'main' });
+  }
+}
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'disallow creating @tracked properties from this.args',
+      category: 'Ember Octane',
+      recommended: false,
+      url: 'https://github.com/ember-cli/eslint-plugin-ember/tree/master/docs/rules/no-tracked-properties-from-args.md',
+    },
+    fixable: null,
+    schema: [],
+    messages: {
+      main: ERROR_MESSAGE,
+    },
+  },
+  create(context) {
+    let trackedImportName;
+
+    return {
+      ImportDeclaration(node) {
+        if (node.source.value === '@glimmer/tracking') {
+          trackedImportName =
+            trackedImportName || getImportIdentifier(node, '@glimmer/tracking', 'tracked');
+        }
+      },
+      ClassProperty(node) {
+        visitClassPropertyOrPropertyDefinition(node, context, trackedImportName);
+      },
+      PropertyDefinition(node) {
+        visitClassPropertyOrPropertyDefinition(node, context, trackedImportName);
+      },
+    };
+  },
+};


Test patch:
diff --git a/tests/lib/rules/no-tracked-properties-from-args.js b/tests/lib/rules/no-tracked-properties-from-args.js
new file mode 100644
index 00000000..9d793793
--- /dev/null
+++ b/tests/lib/rules/no-tracked-properties-from-args.js
@@ -0,0 +1,99 @@
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/no-tracked-properties-from-args');
+const RuleTester = require('eslint').RuleTester;
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester({
+  parser: require.resolve('@babel/eslint-parser'),
+  parserOptions: {
+    ecmaVersion: 2020,
+    sourceType: 'module',
+  },
+});
+
+ruleTester.run('no-tracked-properties-from-args', rule, {
+  valid: [
+    `
+      import { tracked } from '@glimmer/tracking'
+
+      class Test {
+        @someDecorator test = this.args.test
+      }`,
+    `
+      import { tracked } from '@glimmer/tracking'
+
+      class Test {
+        @tracked test = this.someValue
+      }`,
+    `
+      import { tracked } from '@glimmer/tracking'
+
+      class Test {
+        @tracked test = 7
+      }`,
+    `
+      import { tracked } from '@glimmer/tracking'
+      
+      class Test {
+        test = 7
+      }`,
+    `
+      import { tracked } from '@glimmer/tracking'
+
+      class Test {
+        test = "test"
+      }`,
+    `
+      import { tracked } from '@glimmer/tracking'
+
+      class Test {
+        @tracked test = this.notArgs.args.test
+      }`,
+    `
+      import { tracked as fooTracked } from '@glimmer/tracking';
+
+      class Test {
+        @fooTracked test = this.notArgs.args.test
+      }
+    `,
+  ],
+  invalid: [
+    {
+      code: `
+      import { tracked } from '@glimmer/tracking'
+
+      class Test {
+        @tracked test = this.args.test;
+      }`,
+      output: null,
+      errors: [
+        {
+          messageId: 'main',
+          // type could be ClassProperty (ESLint v7) or PropertyDefinition (ESLint v8)
+        },
+      ],
+    },
+    {
+      code: `
+      import { tracked as fooTracked } from '@glimmer/tracking';
+      
+      class Test {
+        @fooTracked test = this.args.test
+      }
+    `,
+      output: null,
+      errors: [
+        {
+          messageId: 'main',
+          // type could be ClassProperty (ESLint v7) or PropertyDefinition (ESLint v8)
+        },
+      ],
+    },
+  ],
+});