feat: Add svelte/prefer-const rule that excludes reactive variables

Author: bfanger

.changeset/tricky-windows-brush.md

@@ -0,0 +1,5 @@
+---
+'eslint-plugin-svelte': minor
+---
+
+Add svelte/prefer-const rule that excludes reactive variables

README.md

@@ -425,6 +425,7 @@ These rules relate to better ways of doing things to help you avoid problems:
 | [svelte/no-unused-class-name](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-unused-class-name/) | disallow the use of a class in the template without a corresponding style |  |
 | [svelte/no-unused-svelte-ignore](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-unused-svelte-ignore/) | disallow unused svelte-ignore comments | :star: |
 | [svelte/no-useless-mustaches](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-useless-mustaches/) | disallow unnecessary mustache interpolations | :wrench: |
+| [svelte/prefer-const](https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-const/) | Require `const` declarations for variables that are never reassigned after declared | :wrench: |
 | [svelte/prefer-destructured-store-props](https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-destructured-store-props/) | destructure values from object stores for better change tracking & fewer redraws | :bulb: |
 | [svelte/require-each-key](https://sveltejs.github.io/eslint-plugin-svelte/rules/require-each-key/) | require keyed `{#each}` block |  |
 | [svelte/require-event-dispatcher-types](https://sveltejs.github.io/eslint-plugin-svelte/rules/require-event-dispatcher-types/) | require type parameters for `createEventDispatcher` |  |

docs/rules.md

@@ -62,6 +62,7 @@ These rules relate to better ways of doing things to help you avoid problems:
 | [svelte/no-unused-class-name](./rules/no-unused-class-name.md)                           | disallow the use of a class in the template without a corresponding style                                                                 |          |
 | [svelte/no-unused-svelte-ignore](./rules/no-unused-svelte-ignore.md)                     | disallow unused svelte-ignore comments                                                                                                    | :star:   |
 | [svelte/no-useless-mustaches](./rules/no-useless-mustaches.md)                           | disallow unnecessary mustache interpolations                                                                                              | :wrench: |
+| [svelte/prefer-const](./rules/prefer-const.md)                                           | Require `const` declarations for variables that are never reassigned after declared                                                       | :wrench: |
 | [svelte/prefer-destructured-store-props](./rules/prefer-destructured-store-props.md)     | destructure values from object stores for better change tracking & fewer redraws                                                          | :bulb:   |
 | [svelte/require-each-key](./rules/require-each-key.md)                                   | require keyed `{#each}` block                                                                                                             |          |
 | [svelte/require-event-dispatcher-types](./rules/require-event-dispatcher-types.md)       | require type parameters for `createEventDispatcher`                                                                                       |          |

docs/rules/prefer-const.md

@@ -0,0 +1,22 @@
+---
+pageClass: 'rule-details'
+sidebarDepth: 0
+title: 'svelte/prefer-const'
+description: 'Require `const` declarations for variables that are never reassigned after declared'
+---
+
+# svelte/prefer-const
+
+> Require `const` declarations for variables that are never reassigned after declared
+
+- :exclamation: <badge text="This rule has not been released yet." vertical="middle" type="error"> **_This rule has not been released yet._** </badge>
+- :wrench: The `--fix` option on the [command line](https://eslint.org/docs/user-guide/command-line-interface#fixing-problems) can automatically fix some of the problems reported by this rule.
+
+## :book: Rule Details
+
+Based on https://eslint.org/docs/latest/rules/prefer-const but skips reactive variables created by runes.
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/src/rules/prefer-const.ts)
+- [Test source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/tests/src/rules/prefer-const.ts)

packages/eslint-plugin-svelte/src/rule-types.ts

@@ -249,6 +249,11 @@ export interface RuleOptions {
    * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-class-directive/
    */
   'svelte/prefer-class-directive'?: Linter.RuleEntry<SveltePreferClassDirective>
+  /**
+   * Require `const` declarations for variables that are never reassigned after declared
+   * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-const/
+   */
+  'svelte/prefer-const'?: Linter.RuleEntry<SveltePreferConst>
   /**
    * destructure values from object stores for better change tracking & fewer redraws
    * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-destructured-store-props/
@@ -460,6 +465,11 @@ type SvelteNoUselessMustaches = []|[{
 type SveltePreferClassDirective = []|[{
   prefer?: ("always" | "empty")
 }]
+// ----- svelte/prefer-const -----
+type SveltePreferConst = []|[{
+  destructuring?: ("any" | "all")
+  ignoreReadBeforeAssign?: boolean
+}]
 // ----- svelte/shorthand-attribute -----
 type SvelteShorthandAttribute = []|[{
   prefer?: ("always" | "never")