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 (excludes reactive values). | :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 (excludes reactive values).                           | :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 (excludes reactive values).'
+---
+
+# svelte/prefer-const
+
+> Require `const` declarations for variables that are never reassigned after declared (excludes reactive values).
+
+- :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/package.json

@@ -61,7 +61,7 @@
     "eslint-compat-utils": "^0.5.1",
     "esutils": "^2.0.3",
     "known-css-properties": "^0.34.0",
-    "postcss": "^8.4.38",
+    "postcss": "^8.4.39",
     "postcss-load-config": "^3.1.4",
     "postcss-safe-parser": "^6.0.0",
     "postcss-selector-parser": "^6.1.0",
@@ -78,27 +78,27 @@
     "@types/esutils": "^2.0.2",
     "@types/json-schema": "^7.0.15",
     "@types/less": "^3.0.6",
-    "@types/mocha": "^10.0.6",
-    "@types/node": "^20.14.2",
+    "@types/mocha": "^10.0.7",
+    "@types/node": "^20.14.10",
     "@types/postcss-safe-parser": "^5.0.4",
     "@types/semver": "^7.5.8",
     "@types/stylus": "^0.48.42",
-    "acorn": "^8.12.0",
+    "acorn": "^8.12.1",
     "assert": "^2.1.0",
     "esbuild": "^0.23.0",
     "esbuild-register": "^3.5.0",
     "eslint-scope": "^8.0.1",
     "eslint-typegen": "^0.2.4",
     "eslint-visitor-keys": "^4.0.0",
-    "espree": "^10.0.1",
+    "espree": "^10.1.0",
     "less": "^4.2.0",
-    "mocha": "^10.4.0",
+    "mocha": "^10.6.0",
     "nyc": "^17.0.0",
     "postcss-nested": "^6.0.1",
-    "sass": "^1.77.5",
+    "sass": "^1.77.6",
     "source-map-js": "^1.2.0",
     "stylus": "^0.63.0",
-    "svelte": "^5.0.0-next.155",
+    "svelte": "5.0.0-next.175",
     "svelte-i18n": "^4.0.0",
     "type-coverage": "^2.29.0",
     "yaml": "^2.4.5"

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 (excludes reactive values).
+   * @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")