feat(linter): implement noUnassignedVariables rule (#6219)

Co-authored-by: Emanuele Stoppa <my.burning@gmail.com>

Author: huangtiandi1999

.changeset/silent-buckets-stop.md

@@ -0,0 +1,23 @@
+---
+"@biomejs/biome": patch
+---
+
+Added new nursery rule [`noUnassignedVariables`](https://biomejs.dev/linter/rules/no-unassigned-variables/), which disallows `let` or `var` variables that are read but never assigned.
+
+The following code is now reported as invalid:
+
+```js
+let x;
+if (x) {
+  console.log(1);
+}
+```
+
+The following code is now reported as valid:
+
+```js
+let x = 1;
+if (x) {
+  console.log(1);
+}
+```

crates/biome_cli/src/execute/migrate/eslint_any_rule_to_biome.rs

@@ -174,6 +174,7 @@ define_categories! {
     "lint/nursery/noShadow": "https://biomejs.dev/linter/rules/no-shadow",
     "lint/nursery/noShorthandPropertyOverrides": "https://biomejs.dev/linter/rules/no-shorthand-property-overrides",
     "lint/nursery/noTsIgnore": "https://biomejs.dev/linter/rules/no-ts-ignore",
+    "lint/nursery/noUnassignedVariables": "https://biomejs.dev/linter/rules/no-unassigned-variables",
     "lint/nursery/noUndeclaredDependencies": "https://biomejs.dev/linter/rules/no-undeclared-dependencies",
     "lint/nursery/noUnknownAtRule": "https://biomejs.dev/linter/rules/no-unknown-at-rule",
     "lint/nursery/noUnknownFunction": "https://biomejs.dev/linter/rules/no-unknown-function",

crates/biome_configuration/src/analyzer/linter/rules.rs

@@ -19,6 +19,7 @@ pub mod no_restricted_elements;
 pub mod no_secrets;
 pub mod no_shadow;
 pub mod no_ts_ignore;
+pub mod no_unassigned_variables;
 pub mod no_unresolved_imports;
 pub mod no_unwanted_polyfillio;
 pub mod no_useless_backref_in_regex;
@@ -43,4 +44,4 @@ pub mod use_single_js_doc_asterisk;
 pub mod use_sorted_classes;
 pub mod use_symbol_description;
 pub mod use_unique_element_ids;
-declare_lint_group! { pub Nursery { name : "nursery" , rules : [self :: no_await_in_loop :: NoAwaitInLoop , self :: no_bitwise_operators :: NoBitwiseOperators , self :: no_constant_binary_expression :: NoConstantBinaryExpression , self :: no_destructured_props :: NoDestructuredProps , self :: no_excessive_lines_per_function :: NoExcessiveLinesPerFunction , self :: no_floating_promises :: NoFloatingPromises , self :: no_global_dirname_filename :: NoGlobalDirnameFilename , self :: no_import_cycles :: NoImportCycles , self :: no_nested_component_definitions :: NoNestedComponentDefinitions , self :: no_noninteractive_element_interactions :: NoNoninteractiveElementInteractions , self :: no_process_global :: NoProcessGlobal , self :: no_react_prop_assign :: NoReactPropAssign , self :: no_restricted_elements :: NoRestrictedElements , self :: no_secrets :: NoSecrets , self :: no_shadow :: NoShadow , self :: no_ts_ignore :: NoTsIgnore , self :: no_unresolved_imports :: NoUnresolvedImports , self :: no_unwanted_polyfillio :: NoUnwantedPolyfillio , self :: no_useless_backref_in_regex :: NoUselessBackrefInRegex , self :: no_useless_escape_in_string :: NoUselessEscapeInString , self :: no_useless_undefined :: NoUselessUndefined , self :: use_adjacent_getter_setter :: UseAdjacentGetterSetter , self :: use_consistent_object_definition :: UseConsistentObjectDefinition , self :: use_consistent_response :: UseConsistentResponse , self :: use_exhaustive_switch_cases :: UseExhaustiveSwitchCases , self :: use_explicit_type :: UseExplicitType , self :: use_exports_last :: UseExportsLast , self :: use_for_component :: UseForComponent , self :: use_google_font_preconnect :: UseGoogleFontPreconnect , self :: use_index_of :: UseIndexOf , self :: use_iterable_callback_return :: UseIterableCallbackReturn , self :: use_json_import_attribute :: UseJsonImportAttribute , self :: use_numeric_separators :: UseNumericSeparators , self :: use_object_spread :: UseObjectSpread , self :: use_parse_int_radix :: UseParseIntRadix , self :: use_readonly_class_properties :: UseReadonlyClassProperties , self :: use_single_js_doc_asterisk :: UseSingleJsDocAsterisk , self :: use_sorted_classes :: UseSortedClasses , self :: use_symbol_description :: UseSymbolDescription , self :: use_unique_element_ids :: UseUniqueElementIds ,] } }
+declare_lint_group! { pub Nursery { name : "nursery" , rules : [self :: no_await_in_loop :: NoAwaitInLoop , self :: no_bitwise_operators :: NoBitwiseOperators , self :: no_constant_binary_expression :: NoConstantBinaryExpression , self :: no_destructured_props :: NoDestructuredProps , self :: no_excessive_lines_per_function :: NoExcessiveLinesPerFunction , self :: no_floating_promises :: NoFloatingPromises , self :: no_global_dirname_filename :: NoGlobalDirnameFilename , self :: no_import_cycles :: NoImportCycles , self :: no_nested_component_definitions :: NoNestedComponentDefinitions , self :: no_noninteractive_element_interactions :: NoNoninteractiveElementInteractions , self :: no_process_global :: NoProcessGlobal , self :: no_react_prop_assign :: NoReactPropAssign , self :: no_restricted_elements :: NoRestrictedElements , self :: no_secrets :: NoSecrets , self :: no_shadow :: NoShadow , self :: no_ts_ignore :: NoTsIgnore , self :: no_unassigned_variables :: NoUnassignedVariables , self :: no_unresolved_imports :: NoUnresolvedImports , self :: no_unwanted_polyfillio :: NoUnwantedPolyfillio , self :: no_useless_backref_in_regex :: NoUselessBackrefInRegex , self :: no_useless_escape_in_string :: NoUselessEscapeInString , self :: no_useless_undefined :: NoUselessUndefined , self :: use_adjacent_getter_setter :: UseAdjacentGetterSetter , self :: use_consistent_object_definition :: UseConsistentObjectDefinition , self :: use_consistent_response :: UseConsistentResponse , self :: use_exhaustive_switch_cases :: UseExhaustiveSwitchCases , self :: use_explicit_type :: UseExplicitType , self :: use_exports_last :: UseExportsLast , self :: use_for_component :: UseForComponent , self :: use_google_font_preconnect :: UseGoogleFontPreconnect , self :: use_index_of :: UseIndexOf , self :: use_iterable_callback_return :: UseIterableCallbackReturn , self :: use_json_import_attribute :: UseJsonImportAttribute , self :: use_numeric_separators :: UseNumericSeparators , self :: use_object_spread :: UseObjectSpread , self :: use_parse_int_radix :: UseParseIntRadix , self :: use_readonly_class_properties :: UseReadonlyClassProperties , self :: use_single_js_doc_asterisk :: UseSingleJsDocAsterisk , self :: use_sorted_classes :: UseSortedClasses , self :: use_symbol_description :: UseSymbolDescription , self :: use_unique_element_ids :: UseUniqueElementIds ,] } }

crates/biome_diagnostics_categories/src/categories.rs

@@ -0,0 +1,137 @@
+use biome_analyze::{Rule, RuleDiagnostic, RuleSource, context::RuleContext, declare_lint_rule};
+use biome_console::markup;
+use biome_js_semantic::ReferencesExtensions;
+use biome_js_syntax::{
+    AnyJsBinding, AnyJsBindingPattern, JsIdentifierBinding, JsVariableDeclaration,
+    JsVariableDeclarationClause, JsVariableDeclarator, JsVariableDeclaratorList,
+    TsDeclareStatement,
+};
+use biome_rowan::AstNode;
+
+use crate::services::semantic::Semantic;
+
+declare_lint_rule! {
+    /// Disallow `let` or `var` variables that are read but never assigned.
+    ///
+    /// This rule flags let or var declarations that are never assigned a value but are still read or used in the code.
+    /// Since these variables will always be undefined, their usage is likely a programming mistake.
+    ///
+    /// ## Examples
+    ///
+    /// ### Invalid
+    ///
+    /// ```js,expect_diagnostic
+    /// let status;
+    /// if (status === 'ready') {
+    ///     console.log('Status is ready');
+    /// }
+    /// ```
+    ///
+    /// ```ts,expect_diagnostic
+    /// let value: number | undefined;
+    /// console.log(value);
+    /// ```
+    ///
+    /// ### Valid
+    ///
+    /// ```js
+    /// let message = "hello";
+    /// console.log(message);
+    ///
+    /// let user;
+    /// user = getUser();
+    /// console.log(user.name);
+    ///
+    /// let count;
+    /// count = 0;
+    /// count++;
+    /// ```
+    ///
+    /// ```ts
+    /// declare let value: number | undefined;
+    /// console.log(value);
+    ///
+    /// declare module "my-module" {
+    ///     let value: string;
+    ///     export = value;
+    /// }
+    /// ```
+    ///
+    pub NoUnassignedVariables {
+        version: "next",
+        name: "noUnassignedVariables",
+        language: "js",
+        sources: &[RuleSource::Eslint("no-unassigned-vars")],
+        recommended: false,
+    }
+}
+
+impl Rule for NoUnassignedVariables {
+    type Query = Semantic<JsVariableDeclarator>;
+    type State = JsIdentifierBinding;
+    type Signals = Option<Self::State>;
+    type Options = ();
+
+    fn run(ctx: &RuleContext<Self>) -> Self::Signals {
+        let declarator = ctx.query();
+        let Ok(AnyJsBindingPattern::AnyJsBinding(AnyJsBinding::JsIdentifierBinding(id))) =
+            declarator.id()
+        else {
+            return None;
+        };
+        let declaration = declarator
+            .parent::<JsVariableDeclaratorList>()?
+            .parent::<JsVariableDeclaration>()?;
+        if declaration.is_const() || declarator.initializer().is_some() {
+            return None;
+        }
+        // e.g. `declare let value: number | undefined;`
+        if declaration
+            .parent::<JsVariableDeclarationClause>()
+            .is_some_and(|clause| clause.parent::<TsDeclareStatement>().is_some())
+        {
+            return None;
+        }
+        // check if the variable is declared in a function or module
+        // e.g. `declare module "my-module" { let value: string; export = value; }`
+        if is_inside_ts_declare_statement(&declaration) {
+            return None;
+        }
+        let model = ctx.model();
+
+        if id.all_writes(model).next().is_some() {
+            return None;
+        }
+        if id.all_reads(model).next().is_some() {
+            return Some(id);
+        }
+
+        None
+    }
+
+    fn diagnostic(ctx: &RuleContext<Self>, state: &Self::State) -> Option<RuleDiagnostic> {
+        // let state = state.to_trimmed_text();
+        let node = ctx.query();
+        let name_token = state.name_token().ok()?;
+        let name = name_token.text_trimmed();
+        Some(
+            RuleDiagnostic::new(
+                rule_category!(),
+                node.range(),
+                markup! {
+                    "The variable '"<Emphasis>{name}</Emphasis>"' is declared but never assigned a value."
+                },
+            )
+            .note(markup! {
+                "Variable declared without assignment. Either assign a value or remove the declaration."
+            }),
+        )
+    }
+}
+
+fn is_inside_ts_declare_statement(node: &JsVariableDeclaration) -> bool {
+    node.syntax()
+        .ancestors()
+        .skip(1)
+        .any(|ancestor| TsDeclareStatement::can_cast(ancestor.kind()))
+}

crates/biome_js_analyze/src/lint/nursery.rs

@@ -0,0 +1,25 @@
+let status;
+if (status === 'ready') {
+  console.log('Ready!');
+}
+
+let user;
+greet(user);
+
+function test() {
+  let error;
+  return error || "Unknown error";
+}
+
+let options;
+const { debug } = options || {};
+
+let flag;
+while (!flag) {
+  // Do something...
+}
+
+let config;
+function init() {
+  return config?.enabled;
+}

crates/biome_js_analyze/src/lint/nursery/no_unassigned_variables.rs

@@ -0,0 +1,132 @@
+---
+source: crates/biome_js_analyze/tests/spec_tests.rs
+expression: invalid.js
+---
+# Input
+```js
+let status;
+if (status === 'ready') {
+  console.log('Ready!');
+}
+
+let user;
+greet(user);
+
+function test() {
+  let error;
+  return error || "Unknown error";
+}
+
+let options;
+const { debug } = options || {};
+
+let flag;
+while (!flag) {
+  // Do something...
+}
+
+let config;
+function init() {
+  return config?.enabled;
+}
+```
+
+# Diagnostics
+```
+invalid.js:1:5 lint/nursery/noUnassignedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  i The variable 'status' is declared but never assigned a value.
+  
+  > 1 │ let status;
+      │     ^^^^^^
+    2 │ if (status === 'ready') {
+    3 │   console.log('Ready!');
+  
+  i Variable declared without assignment. Either assign a value or remove the declaration.
+  
+
+```
+
+```
+invalid.js:6:5 lint/nursery/noUnassignedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  i The variable 'user' is declared but never assigned a value.
+  
+    4 │ }
+    5 │ 
+  > 6 │ let user;
+      │     ^^^^
+    7 │ greet(user);
+    8 │ 
+  
+  i Variable declared without assignment. Either assign a value or remove the declaration.
+  
+
+```
+
+```
+invalid.js:10:7 lint/nursery/noUnassignedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  i The variable 'error' is declared but never assigned a value.
+  
+     9 │ function test() {
+  > 10 │   let error;
+       │       ^^^^^
+    11 │   return error || "Unknown error";
+    12 │ }
+  
+  i Variable declared without assignment. Either assign a value or remove the declaration.
+  
+
+```
+
+```
+invalid.js:14:5 lint/nursery/noUnassignedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  i The variable 'options' is declared but never assigned a value.
+  
+    12 │ }
+    13 │ 
+  > 14 │ let options;
+       │     ^^^^^^^
+    15 │ const { debug } = options || {};
+    16 │ 
+  
+  i Variable declared without assignment. Either assign a value or remove the declaration.
+  
+
+```
+
+```
+invalid.js:17:5 lint/nursery/noUnassignedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  i The variable 'flag' is declared but never assigned a value.
+  
+    15 │ const { debug } = options || {};
+    16 │ 
+  > 17 │ let flag;
+       │     ^^^^
+    18 │ while (!flag) {
+    19 │   // Do something...
+  
+  i Variable declared without assignment. Either assign a value or remove the declaration.
+  
+
+```
+
+```
+invalid.js:22:5 lint/nursery/noUnassignedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  i The variable 'config' is declared but never assigned a value.
+  
+    20 │ }
+    21 │ 
+  > 22 │ let config;
+       │     ^^^^^^
+    23 │ function init() {
+    24 │   return config?.enabled;
+  
+  i Variable declared without assignment. Either assign a value or remove the declaration.
+  
+
+```

crates/biome_js_analyze/src/options.rs

@@ -0,0 +1,2 @@
+let value: number | undefined;
+console.log(value);