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Extract rule: template-no-mut-helper
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‎README.md‎

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@@ -210,6 +210,7 @@ rules in templates can be disabled with eslint directives with mustache or html
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| [template-no-input-placeholder](docs/rules/template-no-input-placeholder.md) | disallow placeholder attribute on input elements | | | |
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| [template-no-input-tagname](docs/rules/template-no-input-tagname.md) | disallow tagName attribute on {{input}} helper | | | |
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| [template-no-log](docs/rules/template-no-log.md) | disallow {{log}} in templates | | | |
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| [template-no-mut-helper](docs/rules/template-no-mut-helper.md) | disallow usage of (mut) helper | | | |
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| [template-no-negated-comparison](docs/rules/template-no-negated-comparison.md) | disallow negated comparisons in templates | | | |
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| [template-no-negated-condition](docs/rules/template-no-negated-condition.md) | disallow negated conditions in if/unless | | | |
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| [template-no-nested-splattributes](docs/rules/template-no-nested-splattributes.md) | disallow nested ...attributes usage | | | |

‎docs/rules/template-no-mut-helper.md‎

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# ember/template-no-mut-helper
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> **HBS Only**: This rule applies to classic `.hbs` template files only (loose mode). It is not relevant for `gjs`/`gts` files (strict mode), where these patterns cannot occur.
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<!-- end auto-generated rule header -->
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Disallow usage of the `(mut)` helper.
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The `(mut)` helper was used in classic Ember to create two-way bindings. In modern Ember (Octane and beyond), this pattern is discouraged in favor of explicit one-way data flow with actions or setters.
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## Rule Details
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This rule disallows using the `(mut)` helper in templates.
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## Reasons to not use [the `mut` helper](https://api.emberjs.com/ember/release/classes/Ember.Templates.helpers/methods/each?anchor=mut)
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1. General problems in the programming model:
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- The mut helper is non-intuitive to use, see, teach, and learn since it can either be a getter or a setter based on the context in which it’s used.
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Example:
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```hbs
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{{#let (mut this.foo) as |foo|}}
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<!-- When used like this, it's a getter -->
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{{foo}}
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<!-- When used like this, it's a setter -->
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<button {{action foo 123}}>Update Foo</button>
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{{/let}}
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```
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- The need for the [no-extra-mut-helper-argument](template-no-extra-mut-helper-argument.md) rule is further evidence that `mut` has a non-intuitive signature and frequently gets misused.
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- The mut helper is usually only used as a pure setter, in which case there are other template helpers that are pure setters that could be used instead of mut (e.g. [ember-set-helper](https://github.com/pzuraq/ember-set-helper)).
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2. Incompatibility with Glimmer Component intentions:
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- The mut helper can re-introduce 2 way data binding into Glimmer Components on named arguments where a child can change a parent’s data, which goes against the Data Down Actions Up principle, goes against Glimmer Components’ intention to have immutable arguments, and is [discouraged by the Ember Core team](https://www.pzuraq.com/blog/on-mut-and-2-way-binding/).
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Example:
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```hbs
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<input type='checkbox' checked={{@checked}} {{on 'change' (fn (mut @checked) (not @checked))}} />
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```
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## What this rule does
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This rule forbids any use of the `mut` helper, both as a getter and a setter, in any context. It also
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surfaces possible alternatives in the lint violation message to help guide engineers to resolving
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the lint violations.
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## Examples
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### Incorrect ❌
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```hbs
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<Input @value={{this.name}} @onChange={{mut this.name}} />
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```
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```hbs
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{{input value=(mut this.name)}}
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```
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```hbs
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<CustomComponent @onChange={{mut this.value}} />
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```
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### Correct ✅
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```hbs
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<Input @value={{this.name}} @onChange={{this.updateName}} />
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```
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```hbs
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<Input @value={{this.name}} @onChange={{fn this.updateName}} />
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```
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```hbs
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<CustomComponent @onChange={{this.handleChange}} />
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```
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## Migration
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1. When used as a pure setter only, `mut` could be replaced by a JS action ("Option 1" below) or [ember-set-helper](https://github.com/pzuraq/ember-set-helper) ("Option 2" below):
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Before:
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```hbs
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<MyComponent @closeDropdown={{action (mut this.setIsDropdownOpen) false}} />
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```
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After (Option 1 HBS):
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```hbs
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<MyComponent @closeDropdown={{action this.setIsDropdownOpen false}} />
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```
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After (Option 1 JS):
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```js
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// in your component class
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class MyComponent extends Component {
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@action
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setIsDropdownOpen(isDropdownOpen) {
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set(this, 'isDropdownOpen', isDropdownOpen);
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}
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}
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```
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After (Option 2):
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```hbs
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<MyComponent @closeDropdown={{set this 'isDropdownOpen' false}} />
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```
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\
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2. When used as a pure getter only, `mut` could be removed:
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Before:
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```hbs
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<MyComponent @isDropdownOpen={{mut this.isDropdownOpen}} />
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```
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After:
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```hbs
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<MyComponent @isDropdownOpen={{this.isDropdownOpen}} />
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```
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\
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3. When `mut` is used as a getter and setter, `mut` could be replaced with a different namespace for the property and a dedicated action function to set the property: (Note: another other option could be to pull in the pick helper from [ember-composable-helpers](https://github.com/DockYard/ember-composable-helpers) and use it like [this](https://github.com/pzuraq/ember-set-helper#picking-values-with-ember-composable-helpers).) (Note: Another option could be to use [ember-box](https://github.com/pzuraq/ember-box)).
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Before:
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```hbs
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{{#let (mut this.foo) as |foo|}}
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{{foo}}
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<input onchange={{action foo value=”target.value”}} />
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{{/let}}
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```
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After HBS:
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```hbs
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{{this.foo}}
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<input {{on “change” this.updateFoo}} />
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```
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After JS:
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```js
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// in your component class
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class MyComponent extends Component {
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@tracked
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foo;
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@action
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updateFoo(evt) {
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this.foo = evt.target.value;
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// or set(this, 'foo', evt.target.value); for legacy Ember code
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}
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}
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```
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\
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4. When `mut` is being passed into a built-in classic component that uses 2 way data binding, `mut` could be removed:
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Before:
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```hbs
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<Input @value={{mut this.profile.description}} />
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```
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After:
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```hbs
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<Input @value={{this.profile.description}} />
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```
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## Options
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| Name | Type | Default | Description |
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| ------------------- | -------- | ------- | ----------------------------------------------------------------------------- |
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| `setterAlternative` | `string` | | If provided, the error message suggests using this helper instead of `(mut)`. |
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## Related Rules
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- [no-mut-helper](./no-mut-helper.md)
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## References
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- [Ember Octane Guide - Two-way bindings](https://guides.emberjs.com/release/upgrading/current-edition/)
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- [eslint-plugin-ember template-no-mut-helper](https://github.com/ember-cli/eslint-plugin-ember/blob/master/docs/rules/template-no-mut-helper.md)

‎lib/rules/template-no-mut-helper.js‎

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/** @type {import('eslint').Rule.RuleModule} */
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module.exports = {
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meta: {
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type: 'suggestion',
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docs: {
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description: 'disallow usage of (mut) helper',
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category: 'Best Practices',
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url: 'https://github.com/ember-cli/eslint-plugin-ember/tree/master/docs/rules/template-no-mut-helper.md',
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templateMode: 'loose',
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},
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fixable: null,
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schema: [
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{
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type: 'object',
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properties: {
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setterAlternative: { type: 'string' },
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},
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additionalProperties: false,
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},
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],
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messages: {
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noMutHelper: '{{message}}',
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},
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originallyFrom: {
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name: 'ember-template-lint',
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rule: 'lib/rules/no-mut-helper.js',
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docs: 'docs/rule/no-mut-helper.md',
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tests: 'test/unit/rules/no-mut-helper-test.js',
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},
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},
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create(context) {
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const options = context.options[0] || {};
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const setterAlternative = options.setterAlternative;
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const message = setterAlternative
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? `Do not use the (mut) helper. Consider using a JS action or {{${setterAlternative}}} instead.`
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: 'Do not use the (mut) helper. Use regular setters or actions instead.';
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function checkNode(node) {
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if (node.path?.type === 'GlimmerPathExpression' && node.path.original === 'mut') {
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context.report({
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node,
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messageId: 'noMutHelper',
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data: { message },
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});
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}
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}
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return {
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GlimmerSubExpression: checkNode,
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GlimmerMustacheStatement: checkNode,
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};
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},
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};

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