feat(lint): support assignment expressions in noParametersOnlyUsedInR…

…ecursion Add support for detecting recursive arrow functions defined via assignment expressions (e.g., foo = (n) => ...). The rule now extracts the function name from the left-hand side of assignment expressions to properly track recursive calls.
biomejs · dyc3 · Oct 31, 2025 · Oct 30, 2025 · Oct 30, 2025 · Oct 30, 2025
commit 8c5950da12500e99f6f0b0d9ea85e2318bf91174
diff --git a/crates/biome_js_analyze/src/lint/nursery/no_parameters_only_used_in_recursion.rs b/crates/biome_js_analyze/src/lint/nursery/no_parameters_only_used_in_recursion.rs
@@ -6,8 +6,8 @@ use biome_console::markup;
 use biome_diagnostics::Severity;
 use biome_js_semantic::ReferencesExtensions;
 use biome_js_syntax::{
-    AnyJsExpression, JsCallExpression, JsIdentifierBinding, JsVariableDeclarator,
-    binding_ext::AnyJsParameterParentFunction,
+    AnyJsExpression, JsAssignmentExpression, JsCallExpression, JsIdentifierBinding,
+    JsVariableDeclarator, binding_ext::AnyJsParameterParentFunction,
 };
 use biome_rowan::{AstNode, BatchMutationExt, TokenText};
 
@@ -264,6 +264,18 @@ fn get_arrow_function_name(
                 .map(|t| t.token_text_trimmed());
         }
 
+        // Check for assignment expression: foo = () => ...
+        if let Some(assignment) = JsAssignmentExpression::cast_ref(&ancestor) {
+            return assignment
+                .left()
+                .ok()?
+                .as_any_js_assignment()?
+                .as_js_identifier_assignment()?
+                .name_token()
+                .ok()
+                .map(|t| t.token_text_trimmed());
+        }
+
         // Stop searching if we hit a function boundary
         // (prevents extracting wrong name from outer scope)
         if is_function_like(&ancestor) {

diff --git a/crates/biome_js_analyze/tests/specs/nursery/noParametersOnlyUsedInRecursion/invalid.js b/crates/biome_js_analyze/tests/specs/nursery/noParametersOnlyUsedInRecursion/invalid.js
@@ -49,3 +49,9 @@ const obj = {
         return this.count(n - step, step);
     }
 };
+
+// Assignment expression with recursive arrow function
+foo = (n, acc) => {
+    if (n === 0) return 0;
+    return foo(n - 1, acc);
+};
diff --git a/crates/biome_js_analyze/tests/specs/nursery/noParametersOnlyUsedInRecursion/invalid.js.snap b/crates/biome_js_analyze/tests/specs/nursery/noParametersOnlyUsedInRecursion/invalid.js.snap
@@ -56,6 +56,12 @@ const obj = {
     }
 };
 
+// Assignment expression with recursive arrow function
+foo = (n, acc) => {
+    if (n === 0) return 0;
+    return foo(n - 1, acc);
+};
+
 ```
 
 # Diagnostics
@@ -329,3 +335,33 @@ invalid.js:47:14 lint/nursery/noParametersOnlyUsedInRecursion  FIXABLE  ━━
 
 
 ```
+
+```
+invalid.js:54:11 lint/nursery/noParametersOnlyUsedInRecursion  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  ! This parameter is only used in recursive calls.
+
+    53 │ // Assignment expression with recursive arrow function
+  > 54 │ foo = (n, acc) => {
+       │           ^^^
+    55 │     if (n === 0) return 0;
+    56 │     return foo(n - 1, acc);
+
+  i Parameters that are only used in recursive calls are effectively unused and can be removed.
+
+  i If the parameter is needed for the recursion to work, consider if the function can be refactored to avoid it.
+
+  i Unsafe fix: If this is intentional, prepend acc with an underscore.
+
+    52 52 │   
+    53 53 │   // Assignment expression with recursive arrow function
+    54    │ - foo·=·(n,·acc)·=>·{
+       54 │ + foo·=·(n,·_acc)·=>·{
+    55 55 │       if (n === 0) return 0;
+    56    │ - ····return·foo(n·-·1,·acc);
+       56 │ + ····return·foo(n·-·1,·_acc);
+    57 57 │   };
+    58 58 │   
+
+
+```
diff --git a/crates/biome_js_analyze/tests/specs/nursery/noParametersOnlyUsedInRecursion/valid.js b/crates/biome_js_analyze/tests/specs/nursery/noParametersOnlyUsedInRecursion/valid.js
@@ -36,3 +36,9 @@ function clamp(n, max) {
     if (n > max) return max;  // max used in comparison
     return clamp(n + 1, max);
 }
+
+// Assignment expression where parameter is used outside recursion
+bar = (n, threshold) => {
+    if (n > threshold) return threshold;
+    return bar(n + 1, threshold);
+};
diff --git a/crates/biome_js_analyze/tests/specs/nursery/noParametersOnlyUsedInRecursion/valid.js.snap b/crates/biome_js_analyze/tests/specs/nursery/noParametersOnlyUsedInRecursion/valid.js.snap
@@ -43,4 +43,10 @@ function clamp(n, max) {
     return clamp(n + 1, max);
 }
 
+// Assignment expression where parameter is used outside recursion
+bar = (n, threshold) => {
+    if (n > threshold) return threshold;
+    return bar(n + 1, threshold);
+};
+
 ```