feat(prefer-let): add rule

.changeset/small-months-exercise.md

@@ -0,0 +1,5 @@
+---
+'eslint-plugin-svelte': major
+---
+
+Adds `prefer-let` rule

README.md

@@ -370,6 +370,7 @@ These rules relate to better ways of doing things to help you avoid problems:
 | [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/prefer-let](https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-let/) | Prefer `let` over `const` for Svelte 5 reactive variable declarations. | :wrench: |
 | [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` |  |
 | [svelte/require-optimized-style-attribute](https://sveltejs.github.io/eslint-plugin-svelte/rules/require-optimized-style-attribute/) | require style attributes that can be optimized |  |

docs/rules.md

@@ -67,6 +67,7 @@ These rules relate to better ways of doing things to help you avoid problems:
 | [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/prefer-let](./rules/prefer-let.md)                                               | Prefer `let` over `const` for Svelte 5 reactive variable declarations.                                                                    | :wrench: |
 | [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`                                                                                       |          |
 | [svelte/require-optimized-style-attribute](./rules/require-optimized-style-attribute.md) | require style attributes that can be optimized                                                                                            |          |

docs/rules/prefer-let.md

@@ -0,0 +1,58 @@
+---
+pageClass: 'rule-details'
+sidebarDepth: 0
+title: 'svelte/prefer-let'
+description: 'Prefer `let` over `const` for Svelte 5 reactive variable declarations.'
+---
+
+# svelte/prefer-let
+
+> Prefer `let` over `const` for Svelte 5 reactive variable declarations.
+
+- :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
+
+This rule reports usages of `const` variable declarations on Svelte reactive
+function assignments. While values may not be reassigned in the code itself,
+they are reassigned by Svelte.
+
+<!--eslint-skip-->
+
+```svelte
+<script>
+  /* eslint svelte/prefer-let: "error" */
+
+  // ✓ GOOD
+  let { a, b } = $props();
+  let c = $state('');
+  let d = $derived(a * 2);
+  let e = $derived.by(() => b * 2);
+
+  // ✗ BAD
+  const g = $state(0);
+  const h = $derived({ count: g });
+</script>
+```
+
+## :wrench: Options
+
+```json
+{
+  "svelte/prefer-const": [
+    "error",
+    {
+      "exclude": ["$props", "$derived", "$derived.by", "$state", "$state.raw"]
+    }
+  ]
+}
+```
+
+- `exclude`: The reactive assignments that you want to exclude from being
+reported..
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/packages/eslint-plugin-svelte/src/rules/prefer-let.ts)
+- [Test source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/packages/eslint-plugin-svelte/tests/src/rules/prefer-let.ts)

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

@@ -285,6 +285,11 @@ export interface RuleOptions {
    * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-destructured-store-props/
    */
   'svelte/prefer-destructured-store-props'?: Linter.RuleEntry<[]>
+  /**
+   * Prefer `let` over `const` for Svelte 5 reactive variable declarations.
+   * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-let/
+   */
+  'svelte/prefer-let'?: Linter.RuleEntry<SveltePreferLet>
   /**
    * require style directives instead of style attribute
    * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-style-directive/
@@ -513,6 +518,10 @@ type SveltePreferConst = []|[{
   destructuring?: ("any" | "all")
   ignoreReadBeforeAssign?: boolean
 }]
+// ----- svelte/prefer-let -----
+type SveltePreferLet = []|[{
+  exclude?: ("$props" | "$derived" | "$derived.by" | "$state" | "$state.raw")[]
+}]
 // ----- svelte/shorthand-attribute -----
 type SvelteShorthandAttribute = []|[{
   prefer?: ("always" | "never")

packages/eslint-plugin-svelte/src/rules/prefer-let.ts

@@ -0,0 +1,90 @@
+import type { TSESTree } from '@typescript-eslint/types';
+
+import { createRule } from '../utils/index.js';
+
+type ReactiveFunction = '$props' | '$derived' | '$derived.by' | '$state' | '$state.raw';
+const DEFAULT_FUNCTIONS: ReactiveFunction[] = [
+	'$props',
+	'$derived',
+	'$derived.by',
+	'$state',
+	'$state.raw'
+];
+
+function getReactiveFunction(callExpr: TSESTree.CallExpression, validNames: string[]) {
+	if (callExpr.callee.type === 'Identifier') {
+		if (validNames.includes(callExpr.callee.name)) {
+			return callExpr.callee.name as ReactiveFunction;
+		}
+	} else if (
+		callExpr.callee.type === 'MemberExpression' &&
+		callExpr.callee.object.type === 'Identifier' &&
+		callExpr.callee.property.type === 'Identifier'
+	) {
+		const fullName = `${callExpr.callee.object.name}.${callExpr.callee.property.name}`;
+
+		if (validNames.includes(fullName)) {
+			return fullName as ReactiveFunction;
+		}
+	}
+
+	return null;
+}
+
+export default createRule('prefer-let', {
+	meta: {
+		docs: {
+			description: 'Prefer `let` over `const` for Svelte 5 reactive variable declarations.',
+			category: 'Best Practices',
+			recommended: false
+		},
+		schema: [
+			{
+				type: 'object',
+				properties: {
+					exclude: {
+						type: 'array',
+						items: {
+							enum: ['$props', '$derived', '$derived.by', '$state', '$state.raw']
+						},
+						uniqueItems: true
+					}
+				},
+				additionalProperties: false
+			}
+		],
+		messages: {
+			'use-let': "'const' is used for a reactive declaration from {{rune}}. Use 'let' instead."
+		},
+		type: 'suggestion',
+		fixable: 'code'
+	},
+	create(context) {
+		const exclude = context.options[0]?.exclude ?? [];
+		const allowedNames = DEFAULT_FUNCTIONS.filter((name) => !exclude.includes(name));
+
+		return {
+			VariableDeclaration(node: TSESTree.VariableDeclaration) {
+				if (node.kind === 'const') {
+					node.declarations.forEach((declarator) => {
+						const init = declarator.init;
+
+						if (!init || init.type !== 'CallExpression') {
+							return;
+						}
+
+						const rune = getReactiveFunction(init, allowedNames);
+						if (rune) {
+							context.report({
+								node,
+								messageId: 'use-let',
+								data: { rune },
+								fix: (fixer) => fixer.replaceTextRange([node.range[0], node.range[0] + 5], 'let')
+							});
+						}
+					});
+				}
+			}
+		};
+	}
+});

packages/eslint-plugin-svelte/src/utils/rules.ts

@@ -56,6 +56,7 @@ import noUselessMustaches from '../rules/no-useless-mustaches.js';
 import preferClassDirective from '../rules/prefer-class-directive.js';
 import preferConst from '../rules/prefer-const.js';
 import preferDestructuredStoreProps from '../rules/prefer-destructured-store-props.js';
+import preferLet from '../rules/prefer-let.js';
 import preferStyleDirective from '../rules/prefer-style-directive.js';
 import requireEachKey from '../rules/require-each-key.js';
 import requireEventDispatcherTypes from '../rules/require-event-dispatcher-types.js';
@@ -127,6 +128,7 @@ export const rules = [
 	preferClassDirective,
 	preferConst,
 	preferDestructuredStoreProps,
+	preferLet,
 	preferStyleDirective,
 	requireEachKey,
 	requireEventDispatcherTypes,

packages/eslint-plugin-svelte/tests/fixtures/rules/prefer-let/invalid/exclude/_config.json

@@ -0,0 +1 @@
+{ "options": [{ "exclude": ["$derived", "$derived.by"] }] }

packages/eslint-plugin-svelte/tests/fixtures/rules/prefer-let/invalid/exclude/exclude-errors.yaml

@@ -0,0 +1,4 @@
+- message: "'const' is used for a reactive declaration from $state. Use 'let' instead."
+  line: 2
+  column: 2
+  suggestions: null

packages/eslint-plugin-svelte/tests/fixtures/rules/prefer-let/invalid/exclude/exclude-input.svelte

@@ -0,0 +1,5 @@
+<script>
+	const a = $state();
+	const b = $derived(a);
+	const c = $derived.by(() => b);
+</script>

packages/eslint-plugin-svelte/tests/fixtures/rules/prefer-let/invalid/exclude/exclude-output.svelte

@@ -0,0 +1,5 @@
+<script>
+	let a = $state();
+	const b = $derived(a);
+	const c = $derived.by(() => b);
+</script>

packages/eslint-plugin-svelte/tests/fixtures/rules/prefer-let/invalid/test01-errors.yaml

@@ -0,0 +1,23 @@
+- message: "'const' is used for a reactive declaration from $props. Use 'let' instead."
+  line: 2
+  column: 2
+  suggestions: null
+- message: "'const' is used for a reactive declaration from $state. Use 'let' instead."
+  line: 3
+  column: 2
+  suggestions: null
+- message: "'const' is used for a reactive declaration from $state.raw. Use 'let'
+    instead."
+  line: 4
+  column: 2
+  suggestions: null
+- message: "'const' is used for a reactive declaration from $derived. Use 'let'
+    instead."
+  line: 5
+  column: 2
+  suggestions: null
+- message: "'const' is used for a reactive declaration from $derived.by. Use 'let'
+    instead."
+  line: 6
+  column: 2
+  suggestions: null

packages/eslint-plugin-svelte/tests/fixtures/rules/prefer-let/invalid/test01-input.svelte

@@ -0,0 +1,7 @@
+<script>
+	const { prop } = $props();
+	const state = $state();
+	const raw = $state.raw();
+	const derived = $derived(state + 1);
+	const derivedBy = $derived.by(() => prop + state);
+</script>

packages/eslint-plugin-svelte/tests/fixtures/rules/prefer-let/invalid/test01-output.svelte

@@ -0,0 +1,7 @@
+<script>
+	let { prop } = $props();
+	let state = $state();
+	let raw = $state.raw();
+	let derived = $derived(state + 1);
+	let derivedBy = $derived.by(() => prop + state);
+</script>

packages/eslint-plugin-svelte/tests/fixtures/rules/prefer-let/valid/test01-input.svelte

@@ -0,0 +1,7 @@
+<script>
+	let { prop } = $props();
+	let state = $state();
+	let raw = $state.raw();
+	let derived = $derived(state + 1);
+	let derivedBy = $derived.by(() => prop + state);
+</script>

packages/eslint-plugin-svelte/tests/src/rules/prefer-const.ts

@@ -1,6 +1,6 @@
-import { RuleTester } from '../../utils/eslint-compat';
-import rule from '../../../src/rules/prefer-const';
-import { loadTestCases } from '../../utils/utils';
+import { RuleTester } from '../../utils/eslint-compat.js';
+import rule from '../../../src/rules/prefer-const.js';
+import { loadTestCases } from '../../utils/utils.js';
 
 const tester = new RuleTester({
 	languageOptions: {

packages/eslint-plugin-svelte/tests/src/rules/prefer-let.ts

@@ -0,0 +1,12 @@
+import { RuleTester } from '../../utils/eslint-compat.js';
+import rule from '../../../src/rules/prefer-let.js';
+import { loadTestCases } from '../../utils/utils.js';
+
+const tester = new RuleTester({
+	languageOptions: {
+		ecmaVersion: 2020,
+		sourceType: 'module'
+	},
+});
+
+tester.run('prefer-let', rule as any, loadTestCases('prefer-let'));