feat(biome_js_analyze): implement noReturnAssign rule #8248

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Merged

dyc3 merged 17 commits into biomejs:main from emilyinure:main

Dec 24, 2025

.changeset/slick-masks-wear.md

-Original file line number
+Diff line change
@@ -0,0 +1,13 @@
+    ---
+    "@biomejs/biome": patch
+    ---
+    Added new nursery rule [`noReturnAssign`](https://biomejs.dev/linter/rules/no-return-assign), which disallows assignments inside return statements.
+    **Invalid:**
+    ```js
+    function f(a) {
+        return a = 1;
+    }
+    ```

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

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

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crates/biome_diagnostics_categories/src/categories.rs

-Original file line number
+Diff line change
@@ Expand Up / @@ -188,6 +188,7 @@ define_categories! { @@
         "lint/nursery/noProto": "https://biomejs.dev/linter/rules/no-proto",
         "lint/nursery/noReactForwardRef": "https://biomejs.dev/linter/rules/no-react-forward-ref",
         "lint/nursery/noScriptUrl": "https://biomejs.dev/linter/rules/no-script-url",
+        "lint/nursery/noReturnAssign": "https://biomejs.dev/linter/rules/no-return-assign",
         "lint/nursery/noShadow": "https://biomejs.dev/linter/rules/no-shadow",
         "lint/nursery/noSyncScripts": "https://biomejs.dev/linter/rules/no-sync-scripts",
         "lint/nursery/noTernary": "https://biomejs.dev/linter/rules/no-ternary",
@@ Expand Down @@

crates/biome_js_analyze/src/lint/nursery.rs

-Original file line number
+Diff line change
@@ Expand Up / @@ -22,6 +22,7 @@ pub mod no_next_async_client_component; @@
     pub mod no_parameters_only_used_in_recursion;
     pub mod no_proto;
     pub mod no_react_forward_ref;
+    pub mod no_return_assign;
     pub mod no_script_url;
     pub mod no_shadow;
     pub mod no_sync_scripts;
@@ Expand Down Expand Up / @@ -54,4 +55,4 @@ pub mod use_spread; @@
     pub mod use_vue_consistent_define_props_declaration;
     pub mod use_vue_define_macros_order;
     pub mod use_vue_multi_word_component_names;
-    declare_lint_group! { pub Nursery { name : "nursery" , rules : [self :: no_ambiguous_anchor_text :: NoAmbiguousAnchorText , self :: no_continue :: NoContinue , self :: no_deprecated_imports :: NoDeprecatedImports , self :: no_duplicated_spread_props :: NoDuplicatedSpreadProps , self :: no_empty_source :: NoEmptySource , self :: no_equals_to_null :: NoEqualsToNull , self :: no_floating_promises :: NoFloatingPromises , self :: no_for_in :: NoForIn , self :: no_import_cycles :: NoImportCycles , self :: no_increment_decrement :: NoIncrementDecrement , self :: no_jsx_literals :: NoJsxLiterals , self :: no_leaked_render :: NoLeakedRender , self :: no_misused_promises :: NoMisusedPromises , self :: no_multi_assign :: NoMultiAssign , self :: no_multi_str :: NoMultiStr , self :: no_next_async_client_component :: NoNextAsyncClientComponent , self :: no_parameters_only_used_in_recursion :: NoParametersOnlyUsedInRecursion , self :: no_proto :: NoProto , self :: no_react_forward_ref :: NoReactForwardRef , self :: no_script_url :: NoScriptUrl , self :: no_shadow :: NoShadow , self :: no_sync_scripts :: NoSyncScripts , self :: no_ternary :: NoTernary , self :: no_undeclared_env_vars :: NoUndeclaredEnvVars , self :: no_unknown_attribute :: NoUnknownAttribute , self :: no_unnecessary_conditions :: NoUnnecessaryConditions , self :: no_unresolved_imports :: NoUnresolvedImports , self :: no_unused_expressions :: NoUnusedExpressions , self :: no_useless_catch_binding :: NoUselessCatchBinding , self :: no_useless_undefined :: NoUselessUndefined , self :: no_vue_data_object_declaration :: NoVueDataObjectDeclaration , self :: no_vue_duplicate_keys :: NoVueDuplicateKeys , self :: no_vue_reserved_keys :: NoVueReservedKeys , self :: no_vue_reserved_props :: NoVueReservedProps , self :: no_vue_setup_props_reactivity_loss :: NoVueSetupPropsReactivityLoss , self :: use_array_sort_compare :: UseArraySortCompare , self :: use_await_thenable :: UseAwaitThenable , self :: use_consistent_arrow_return :: UseConsistentArrowReturn , self :: use_destructuring :: UseDestructuring , self :: use_exhaustive_switch_cases :: UseExhaustiveSwitchCases , self :: use_explicit_type :: UseExplicitType , self :: use_find :: UseFind , self :: use_max_params :: UseMaxParams , self :: use_qwik_method_usage :: UseQwikMethodUsage , self :: use_qwik_valid_lexical_scope :: UseQwikValidLexicalScope , self :: use_regexp_exec :: UseRegexpExec , self :: use_sorted_classes :: UseSortedClasses , self :: use_spread :: UseSpread , self :: use_vue_consistent_define_props_declaration :: UseVueConsistentDefinePropsDeclaration , self :: use_vue_define_macros_order :: UseVueDefineMacrosOrder , self :: use_vue_multi_word_component_names :: UseVueMultiWordComponentNames ,] } }
+    declare_lint_group! { pub Nursery { name : "nursery" , rules : [self :: no_ambiguous_anchor_text :: NoAmbiguousAnchorText , self :: no_continue :: NoContinue , self :: no_deprecated_imports :: NoDeprecatedImports , self :: no_duplicated_spread_props :: NoDuplicatedSpreadProps , self :: no_empty_source :: NoEmptySource , self :: no_equals_to_null :: NoEqualsToNull , self :: no_floating_promises :: NoFloatingPromises , self :: no_for_in :: NoForIn , self :: no_import_cycles :: NoImportCycles , self :: no_increment_decrement :: NoIncrementDecrement , self :: no_jsx_literals :: NoJsxLiterals , self :: no_leaked_render :: NoLeakedRender , self :: no_misused_promises :: NoMisusedPromises , self :: no_multi_assign :: NoMultiAssign , self :: no_multi_str :: NoMultiStr , self :: no_next_async_client_component :: NoNextAsyncClientComponent , self :: no_parameters_only_used_in_recursion :: NoParametersOnlyUsedInRecursion , self :: no_proto :: NoProto , self :: no_react_forward_ref :: NoReactForwardRef , self :: no_return_assign :: NoReturnAssign , self :: no_script_url :: NoScriptUrl , self :: no_shadow :: NoShadow , self :: no_sync_scripts :: NoSyncScripts , self :: no_ternary :: NoTernary , self :: no_undeclared_env_vars :: NoUndeclaredEnvVars , self :: no_unknown_attribute :: NoUnknownAttribute , self :: no_unnecessary_conditions :: NoUnnecessaryConditions , self :: no_unresolved_imports :: NoUnresolvedImports , self :: no_unused_expressions :: NoUnusedExpressions , self :: no_useless_catch_binding :: NoUselessCatchBinding , self :: no_useless_undefined :: NoUselessUndefined , self :: no_vue_data_object_declaration :: NoVueDataObjectDeclaration , self :: no_vue_duplicate_keys :: NoVueDuplicateKeys , self :: no_vue_reserved_keys :: NoVueReservedKeys , self :: no_vue_reserved_props :: NoVueReservedProps , self :: no_vue_setup_props_reactivity_loss :: NoVueSetupPropsReactivityLoss , self :: use_array_sort_compare :: UseArraySortCompare , self :: use_await_thenable :: UseAwaitThenable , self :: use_consistent_arrow_return :: UseConsistentArrowReturn , self :: use_destructuring :: UseDestructuring , self :: use_exhaustive_switch_cases :: UseExhaustiveSwitchCases , self :: use_explicit_type :: UseExplicitType , self :: use_find :: UseFind , self :: use_max_params :: UseMaxParams , self :: use_qwik_method_usage :: UseQwikMethodUsage , self :: use_qwik_valid_lexical_scope :: UseQwikValidLexicalScope , self :: use_regexp_exec :: UseRegexpExec , self :: use_sorted_classes :: UseSortedClasses , self :: use_spread :: UseSpread , self :: use_vue_consistent_define_props_declaration :: UseVueConsistentDefinePropsDeclaration , self :: use_vue_define_macros_order :: UseVueDefineMacrosOrder , self :: use_vue_multi_word_component_names :: UseVueMultiWordComponentNames ,] } }

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

-Original file line number
+Diff line change
@@ -0,0 +1,125 @@
+    use biome_analyze::{
+        QueryMatch, Rule, RuleDiagnostic, RuleSource, context::RuleContext, declare_lint_rule,
+    };
+    use biome_console::markup;
+    use biome_diagnostics::Severity;
+    use biome_js_syntax::{
+        AnyJsExpression, JsArrowFunctionExpression, JsAssignmentExpression, JsClassExpression,
+        JsFunctionExpression, JsReturnStatement,
+    };
+    use biome_rowan::{AstNode, TextRange, WalkEvent, declare_node_union};
+    use biome_rule_options::no_return_assign::NoReturnAssignOptions;
+    use crate::services::semantic::Semantic;
+    declare_lint_rule! {
+        /// Disallow assignments in return statements.
+        ///
+        /// In return statements, it is common to mistype a comparison operator (such as `==`) as an assignment operator (such as `=`).
+        /// Moreover, the use of assignments in a return statement is confusing.
+        /// Return statements are often considered side-effect free.
+        ///
+        /// ## Examples
+        ///
+        /// ### Invalid
+        ///
+        /// ```js,expect_diagnostic
+        /// function f(a) {
+        ///     return a = 1;
+        /// }
+        /// ```
+        ///
+        /// ### Valid
+        ///
+        /// ```js
+        /// function f(a) {
+        ///     a = 1;
+        ///     return a;
+        /// }
+        /// ```
+        ///
+        /// ```js
+        /// function f(a) {
+        ///     return a == 1;
+        /// }
+        /// ```
+        ///
+        pub NoReturnAssign {
+            version: "next",
+            name: "noReturnAssign",
+            language: "js",
+            sources: &[RuleSource::Eslint("no-return-assign").same()],
+            recommended: false,
+            severity: Severity::Error,
+        }
+    }
+    declare_node_union! {
+        pub AnyReturn = JsReturnStatement | JsArrowFunctionExpression
+    }
+    impl Rule for NoReturnAssign {
+        type Query = Semantic<AnyReturn>;
+        type State = TextRange;
+        type Signals = Vec<Self::State>;
+        type Options = NoReturnAssignOptions;
+        fn run(ctx: &RuleContext<Self>) -> Self::Signals {
+            run_options(ctx).unwrap_or_default()
+        }
+        fn diagnostic(_ctx: &RuleContext<Self>, state: &Self::State) -> Option<RuleDiagnostic> {
+            Some(
+                RuleDiagnostic::new(
+                    rule_category!(),
+                    state,
+                    markup!{
+                        <Emphasis>"Return statements"</Emphasis>" should not contain "<Emphasis>"assignments"</Emphasis>"."
+                    },
+                ).note(markup! {
+                    "Assignments inside return statements are easy to mistake for comparison operators (`==`), "
+                    "and add unexpected side effects to normally-pure code."
+                    "\nIf the assignment is intentional, move it outside of the return statement."
+                }))
+        }
+    }
+    fn run_options(ctx: &RuleContext<NoReturnAssign>) -> Option<Vec<TextRange>> {
+        match ctx.query() {
+            AnyReturn::JsReturnStatement(query) => Some(traverse_expression(&query.argument()?)),
+            AnyReturn::JsArrowFunctionExpression(query) => Some(traverse_expression(
+                query.body().ok()?.as_any_js_expression()?,
+            )),
+        }
+    }
+    fn traverse_expression(root: &AnyJsExpression) -> Vec<TextRange> {
+        let mut signal = Vec::new();
+        let mut iter = root.syntax().preorder();
+        while let Some(event) = iter.next() {
+            if let WalkEvent::Enter(node) = event {
+                if JsAssignmentExpression::can_cast(node.kind()) {
+                    signal.push(node.text_range());
+                }
+                // Skip function and class boundaries - assignments inside nested
+                // functions/classes are not part of this return statement
+                if JsArrowFunctionExpression::can_cast(node.kind())
+                    || JsFunctionExpression::can_cast(node.kind())
+                    || JsClassExpression::can_cast(node.kind())
+                {
+                    iter.skip_subtree();
+                    continue;
+                }
+                let is_expression = AnyJsExpression::can_cast(node.kind());
+                if !is_expression {
+                    iter.skip_subtree();
+                }
+            }
+        }
+        signal
+    }

crates/biome_js_analyze/tests/specs/nursery/noReturnAssign/invalid.js

-Original file line number
+Diff line change
@@ -0,0 +1,110 @@
+    /* should generate diagnostics */
+    const f = (a) => a = 1;
+    function f(a) {
+    	return a = 1;
+    }
+    function f(a) {
+    	return (a = 1);
+    }
+    function f(a, b, c) {
+    	return (a, b, c = 1);
+    }
+    function f(a, b, c) {
+    	return a == (b = c);
+    }
+    function f(a, b) {
+    	return a && (b = 1);
+    }
+    function f(a, b, c) {
+    	return a = (b = c);
+    }
+    function f(a, b, c) {
+    	return (a = 1, b = 2, c = 3);
+    }
+    function f(a) {
+    	return [a = 1];
+    }
+    async function f(a) {
+    	return await (a = 1);
+    }
+    function f(a) {
+    	return 5 + (a = 1);
+    }
+    function f(a) {
+    	return (a = 1) + 5;
+    }
+    function f(a) {
+    	return foo(a = 1);
+    }
+    function f(a, b) {
+    	return b[a = 1];
+    }
+    function f(a) {
+    	return (a = 1) ? true : false;
+    }
+    function f(a) {
+    	return true ? a = 1 : false;
+    }
+    function f(a, b) {
+    	return true ? false : a = 1;
+    }
+    function f(a, b) {
+    	return (a = 1) in b;
+    }
+    function f(a, Class) {
+    	return (a = 1) instanceof Class;
+    }
+    function f(a, b) {
+    	return (a = 1) || b;
+    }
+    function f(a, b) {
+    	return a || (b = 1);
+    }
+    function f(a, b) {
+    	return (a = 1) ?? b;
+    }
+    function f(a, b) {
+    	return a ?? (b = 1);
+    }
+    function f(a) {
+    	return !(a = 1);
+    }
+    function f(a) {
+    	return typeof (a = 1);
+    }
+    function f(a) {
+    	return void (a = 1);
+    }
+    function f(a) {
+    	return -(a = 1);
+    }
+    function f(a) {
+    	return <div prop={a = 1} />;
+    }

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