feat: add svelte/no-reactive-reassign rule

.changeset/tidy-pugs-draw.md

@@ -0,0 +1,5 @@
+---
+"eslint-plugin-svelte": minor
+---
+
+feat: add `svelte/no-reactive-reassign` rule

README.md

@@ -319,6 +319,7 @@ These rules relate to possible syntax or logic errors in Svelte code:
 | [svelte/no-export-load-in-svelte-module-in-kit-pages](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-export-load-in-svelte-module-in-kit-pages/) | disallow exporting load functions in `*.svelte` module in Svelte Kit page components. |  |
 | [svelte/no-not-function-handler](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-not-function-handler/) | disallow use of not function in event handler | :star: |
 | [svelte/no-object-in-text-mustaches](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-object-in-text-mustaches/) | disallow objects in text mustache interpolation | :star: |
+| [svelte/no-reactive-reassign](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-reactive-reassign/) | disallow reassigning reactive values |  |
 | [svelte/no-shorthand-style-property-overrides](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-shorthand-style-property-overrides/) | disallow shorthand style properties that override related longhand properties | :star: |
 | [svelte/no-store-async](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-store-async/) | disallow using async/await inside svelte stores because it causes issues with the auto-unsubscribing features |  |
 | [svelte/no-unknown-style-directive-property](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-unknown-style-directive-property/) | disallow unknown `style:property` | :star: |

docs/rules.md

@@ -26,6 +26,7 @@ These rules relate to possible syntax or logic errors in Svelte code:
 | [svelte/no-export-load-in-svelte-module-in-kit-pages](./rules/no-export-load-in-svelte-module-in-kit-pages.md) | disallow exporting load functions in `*.svelte` module in Svelte Kit page components. |  |
 | [svelte/no-not-function-handler](./rules/no-not-function-handler.md) | disallow use of not function in event handler | :star: |
 | [svelte/no-object-in-text-mustaches](./rules/no-object-in-text-mustaches.md) | disallow objects in text mustache interpolation | :star: |
+| [svelte/no-reactive-reassign](./rules/no-reactive-reassign.md) | disallow reassigning reactive values |  |
 | [svelte/no-shorthand-style-property-overrides](./rules/no-shorthand-style-property-overrides.md) | disallow shorthand style properties that override related longhand properties | :star: |
 | [svelte/no-store-async](./rules/no-store-async.md) | disallow using async/await inside svelte stores because it causes issues with the auto-unsubscribing features |  |
 | [svelte/no-unknown-style-directive-property](./rules/no-unknown-style-directive-property.md) | disallow unknown `style:property` | :star: |

docs/rules/no-reactive-reassign.md

@@ -0,0 +1,122 @@
+---
+pageClass: "rule-details"
+sidebarDepth: 0
+title: "svelte/no-reactive-reassign"
+description: "disallow reassigning reactive values"
+---
+
+# svelte/no-reactive-reassign
+
+> disallow reassigning reactive values
+
+- :exclamation: <badge text="This rule has not been released yet." vertical="middle" type="error"> **_This rule has not been released yet._** </badge>
+
+## :book: Rule Details
+
+This rule aims to prevent unintended behavior caused by modification or reassignment of reactive values.
+
+<ESLintCodeBlock>
+
+<!--eslint-skip-->
+
+```svelte
+<script>
+  /* eslint svelte/no-reactive-reassign: "error" */
+  let value = 0
+  $: reactiveValue = value * 2
+
+  function handleClick() {
+    /* ✓ GOOD */
+    value++
+    /* ✗ BAD */
+    reactiveValue = value * 3
+    reactiveValue++
+  }
+</script>
+
+<!-- ✓ GOOD -->
+<input type="number" bind:value />
+<!-- ✗ BAD -->
+<input type="number" bind:value={reactiveValue} />
+```
+
+</ESLintCodeBlock>
+
+## :wrench: Options
+
+```json
+{
+  "svelte/no-reactive-reassign": ["error", {
+    "props": true
+  }]
+}
+```
+
+- `props` ... If set to `true`, this rule warns against the modification of reactive value properties. Default is `true`.
+
+### `{ "props": true }`
+
+<ESLintCodeBlock>
+
+<!--eslint-skip-->
+
+```svelte
+<script>
+  /* eslint svelte/no-reactive-reassign: ["error", { "props": true }] */
+  let value = 0
+  $: reactiveValue = { value: value * 2}
+
+  function handleClick() {
+    /* ✓ GOOD */
+    value++
+    /* ✗ BAD */
+    reactiveValue.value++
+    reactiveValue = { value: reactiveValue.value + 1 }
+  }
+</script>
+
+<!-- ✓ GOOD -->
+<input type="number" bind:value />
+<!-- ✗ BAD -->
+<input type="number" bind:value={reactiveValue.value} />
+<MyComponent bind:objectValue={reactiveValue} />
+```
+
+</ESLintCodeBlock>
+
+### `{ "props": false }`
+
+<ESLintCodeBlock>
+
+<!--eslint-skip-->
+
+```svelte
+<script>
+  /* eslint svelte/no-reactive-reassign: ["error", { "props": false }] */
+  let value = 0
+  $: reactiveValue = { value: value * 2}
+
+  function handleClick() {
+    /* ✓ GOOD */
+    value++
+    /* OK */
+    reactiveValue.value++
+    /* ✗ BAD */
+    reactiveValue = { value: reactiveValue.value + 1 }
+  }
+</script>
+
+<!-- ✓ GOOD -->
+<input type="number" bind:value />
+<!-- OK -->
+<input type="number" bind:value={reactiveValue.value} />
+<!-- ✗ BAD -->
+<MyComponent bind:objectValue={reactiveValue} />
+```
+
+</ESLintCodeBlock>
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/src/rules/no-reactive-reassign.ts)
+- [Test source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/tests/src/rules/no-reactive-reassign.ts)