fix: only flag object assign in accumulators, if it is allocating a new object for each iter (#7078)

Co-authored-by: Carson McManus <[email protected]>

Author: emilyinure

.changeset/wet-jeans-repair.md

@@ -0,0 +1,20 @@
+---
+"@biomejs/biome": patch
+---
+
+Fixed [#6675](https://github.com/biomejs/biome/issues/6675): Now only flags
+noAccumulatingSpread on Object.assign when a new object is being allocated on
+each iteration. Before, all cases using Object.assign with reduce parameters
+were warned despite not making new allocations.
+
+The following code will no longer be a false positive:
+
+```js
+foo.reduce((acc, bar) => Object.assign(acc, bar), {})
+```
+
+The following cases which **do** make new allocations will continue to warn:
+
+```js
+foo.reduce((acc, bar) => Object.assign({}, acc, bar), {})
+```

crates/biome_js_analyze/src/lint/performance/no_accumulating_spread.rs

@@ -43,19 +43,24 @@ declare_lint_rule! {
     ///
     /// ```js,expect_diagnostic
     /// var a = ['a', 'b', 'c'];
-    /// a.reduce((acc, val) => Object.assign(acc, val), []);
+    /// a.reduce((acc, val) => {return Object.assign([], acc, val);}, []);
     /// ```
     ///
-    /// ```js,expect_diagnostic
+    /// ### Valid
+    ///
+    /// ```js
     /// var a = ['a', 'b', 'c'];
-    /// a.reduce((acc, val) => {return Object.assign(acc, val);}, []);
+    /// a.reduce((acc, val) => {acc.push(val); return acc}, []);
     /// ```
     ///
-    /// ### Valid
+    /// ```js
+    /// var a = ['a', 'b', 'c'];
+    /// a.reduce((acc, val) => Object.assign(acc, val), []);
+    /// ```
     ///
     /// ```js
     /// var a = ['a', 'b', 'c'];
-    /// a.reduce((acc, val) => {acc.push(val); return acc}, []);
+    /// a.reduce((acc, val) => {return Object.assign(acc, val);}, []);
     /// ```
     ///
     pub NoAccumulatingSpread {
@@ -200,9 +205,21 @@ fn handle_object_assign(node: &JsStaticMemberExpression, model: &SemanticModel)
 
     let call_expression = node.parent::<JsCallExpression>()?;
     let arguments = call_expression.arguments().ok()?;
-    let reference = arguments
-        .args()
-        .first()?
+
+    let mut arg_iter = arguments.args().iter();
+
+    let first = arg_iter.next()?.ok()?;
+    let first = first.as_any_js_expression()?;
+
+    if first.as_js_object_expression().is_none()
+        && first.as_js_array_expression().is_none()
+        && first.as_js_new_expression().is_none()
+    {
+        return None;
+    }
+
+    let reference = arg_iter
+        .next()?
         .ok()?
         .as_any_js_expression()?
         .as_js_identifier_expression()?

crates/biome_js_analyze/tests/specs/performance/noAccumulatingSpread/invalid.jsonc

@@ -15,13 +15,6 @@
 	"foo.reduce((acc, bar) => {return [...acc, ...bar];}, [])",
 	"foo.reduceRight((acc, bar) => {return [...acc, ...bar];}, [])",
 
-	// Array - Arrow return with assign
-	"foo.reduce((acc, bar) => Object.assign(acc, bar), [])",
-	"foo.reduceRight((acc, bar) => Object.assign(acc, bar), [])",
-
-	// Array - Body return with Assign
-	"foo.reduce((acc, bar) => { return Object.assign(acc, bar) }, [])",
-	"foo.reduceRight((acc, bar) => { return Object.assign(acc, bar) }, [])",
 
 	// Object - Arrow return
 	"foo.reduce((acc, bar) => ({...acc, [bar.key]: bar.value}), {})",
@@ -39,11 +32,27 @@
 	"foo.reduce((acc, bar) => {return {...acc, ...bar};}, {})",
 	"foo.reduceRight((acc, bar) => {return {...acc, ...bar};}, {})",
 
+	// Array - Arrow return with assign
+	"foo.reduce((acc, bar) => Object.assign({}, acc, bar), [])",
+	"foo.reduceRight((acc, bar) => Object.assign({}, acc, bar), [])",
+
+	// Array - Body return with Assign
+	"foo.reduce((acc, bar) => { return Object.assign([], acc, bar) }, [])",
+	"foo.reduceRight((acc, bar) => { return Object.assign([], acc, bar) }, [])",
+
 	// Object - Arrow return with assign
-	"foo.reduce((acc, bar) => Object.assign(acc, bar), {})",
-	"foo.reduceRight((acc, bar) => Object.assign(acc, bar), {})",
+	"foo.reduce((acc, bar) => Object.assign([], acc, bar), {})",
+	"foo.reduceRight((acc, bar) => Object.assign([], acc, bar), {})",
 
 	// Object - Body return with Assign
-	"foo.reduce((acc, bar) => { return Object.assign(acc, bar) }, {})",
-	"foo.reduceRight((acc, bar) => { return Object.assign(acc, bar) }, {})"
+	"foo.reduce((acc, bar) => { return Object.assign([], acc, bar) }, {})",
+	"foo.reduceRight((acc, bar) => { return Object.assign([], acc, bar) }, {})",
+
+	// Array - Body return with Assign new
+	"foo.reduce((acc, bar) => { return Object.assign(new Array(1), acc, bar) }, [])",
+	"foo.reduceRight((acc, bar) => { return Object.assign(new Array(1), acc, bar) }, [])",
+
+	// Object - Arrow return with assign new
+	"foo.reduce((acc, bar) => Object.assign(new Array(1), acc, bar), {})",
+	"foo.reduceRight((acc, bar) => Object.assign(new Array(1), acc, bar), {})"
 ]