fix(minifier): keep variables that are modified by combined assignments made by minification

crates/oxc_minifier/src/peephole/minimize_conditions.rs

@@ -4,6 +4,7 @@ use oxc_ecmascript::{
     constant_evaluation::{ConstantEvaluation, ConstantValue, DetermineValueType},
     side_effects::MayHaveSideEffects,
 };
+use oxc_semantic::ReferenceFlags;
 use oxc_span::GetSpan;
 use oxc_syntax::es_target::ESTarget;
 
@@ -200,6 +201,9 @@ impl<'a> PeepholeOptimizations {
             return;
         }
 
+        let reference = ctx.scoping_mut().get_reference_mut(write_id_ref.reference_id());
+        reference.flags_mut().insert(ReferenceFlags::Read);
+
         let new_op = logical_expr.operator.to_assignment_operator();
         expr.operator = new_op;
         expr.right = logical_expr.right.take_in(ctx.ast);
@@ -220,6 +224,9 @@ impl<'a> PeepholeOptimizations {
         {
             return;
         }
+
+        Self::mark_assignment_target_as_read(&expr.left, ctx);
+
         expr.operator = new_op;
         expr.right = binary_expr.right.take_in(ctx.ast);
         ctx.state.changed = true;

crates/oxc_minifier/src/peephole/minimize_logical_expression.rs

@@ -1,5 +1,6 @@
 use oxc_allocator::TakeIn;
 use oxc_ast::ast::*;
+use oxc_semantic::ReferenceFlags;
 use oxc_span::{ContentEq, GetSpan};
 use oxc_syntax::es_target::ESTarget;
 
@@ -236,6 +237,8 @@ impl<'a> PeepholeOptimizations {
                 unreachable!()
             };
 
+            Self::mark_assignment_target_as_read(&assignment_expr.left, ctx);
+
             let assign_value = assignment_expr.right.take_in(ctx.ast);
             sequence_expr.expressions.push(assign_value);
             *expr = ctx.ast.expression_assignment(
@@ -259,6 +262,9 @@ impl<'a> PeepholeOptimizations {
         {
             return;
         }
+
+        Self::mark_assignment_target_as_read(&assignment_expr.left, ctx);
+
         let span = e.span;
         let Expression::AssignmentExpression(assignment_expr) = &mut e.right else {
             return;
@@ -268,4 +274,15 @@ impl<'a> PeepholeOptimizations {
         *expr = e.right.take_in(ctx.ast);
         ctx.state.changed = true;
     }
+
+    /// Marks the AssignmentTargetIdentifier of assignment expressions as ReferenceFlags::Read
+    ///
+    /// When creating AssignmentTargetIdentifier from normal expressions, the identifier only has ReferenceFlags::Write.
+    /// But assignment expressions changes the value, so we should add ReferenceFlags::Read.
+    pub fn mark_assignment_target_as_read(assign_target: &AssignmentTarget, ctx: &mut Ctx<'a, '_>) {
+        if let AssignmentTarget::AssignmentTargetIdentifier(id) = assign_target {
+            let reference = ctx.scoping_mut().get_reference_mut(id.reference_id());
+            reference.flags_mut().insert(ReferenceFlags::Read);
+        }
+    }
 }

crates/oxc_minifier/tests/peephole/oxc.rs

@@ -1,6 +1,13 @@
+use oxc_minifier::CompressOptions;
+
+use super::super::test as test_options;
 /// Oxc Integration Tests
 use super::{test, test_same};
 
+fn test_unused(source_text: &str, expected: &str) {
+    test_options(source_text, expected, CompressOptions::default());
+}
+
 #[test]
 fn integration() {
     test(
@@ -104,3 +111,71 @@ fn eval() {
     test_same("eval?.(x, y)");
     test_same("eval?.(x,y)");
 }
+
+#[test]
+fn unused() {
+    test_unused(
+        "(function() {
+          let v;
+          window.foo = function() {
+            return v ?? (v = bar());
+          }
+        })()
+        ",
+        "(function() {
+          let v;
+          window.foo = function() {
+            return v ??= bar();
+          }
+        })()
+        ",
+    );
+    test_unused(
+        "(function() {
+          let v;
+          window.foo = function() {
+            return v ?? (console.log(), v = bar());
+          }
+        })()
+        ",
+        "(function() {
+          let v;
+          window.foo = function() {
+            return v ??= (console.log(), bar());
+          }
+        })()
+        ",
+    );
+    test_unused(
+        "(function() {
+          let v;
+          window.foo = function() {
+            return v = v || bar();
+          }
+        })()
+        ",
+        "(function() {
+          let v;
+          window.foo = function() {
+            return v ||= bar();
+          }
+        })()
+        ",
+    );
+    test_unused(
+        "(function() {
+          let v;
+          window.foo = function() {
+            return v = v + bar();
+          }
+        })()
+        ",
+        "(function() {
+          let v;
+          window.foo = function() {
+            return v += bar();
+          }
+        })()
+        ",
+    );
+}