diff --git a/crates/oxc_ecmascript/src/constant_evaluation/value_type.rs b/crates/oxc_ecmascript/src/constant_evaluation/value_type.rs
index 504460c67ac8d..9ff470aa55e21 100644
--- a/crates/oxc_ecmascript/src/constant_evaluation/value_type.rs
+++ b/crates/oxc_ecmascript/src/constant_evaluation/value_type.rs
@@ -1,4 +1,7 @@
-use oxc_ast::ast::{BinaryExpression, ConditionalExpression, Expression, LogicalExpression};
+use oxc_ast::ast::{
+    AssignmentExpression, AssignmentOperator, BinaryExpression, ConditionalExpression, Expression,
+    LogicalExpression,
+};
 use oxc_syntax::operator::{BinaryOperator, UnaryOperator};
 
 /// JavaScript Language Type
@@ -97,7 +100,7 @@ impl<'a> From<&Expression<'a>> for ValueType {
             Expression::SequenceExpression(e) => {
                 e.expressions.last().map_or(ValueType::Undetermined, Self::from)
             }
-            Expression::AssignmentExpression(e) => Self::from(&e.right),
+            Expression::AssignmentExpression(e) => Self::from(&**e),
             Expression::ConditionalExpression(e) => Self::from(&**e),
             Expression::LogicalExpression(e) => Self::from(&**e),
             _ => Self::Undetermined,
@@ -167,6 +170,45 @@ impl<'a> From<&BinaryExpression<'a>> for ValueType {
     }
 }
 
+impl<'a> From<&AssignmentExpression<'a>> for ValueType {
+    fn from(e: &AssignmentExpression<'a>) -> Self {
+        match e.operator {
+            AssignmentOperator::Assign => Self::from(&e.right),
+            AssignmentOperator::Addition => {
+                let right = Self::from(&e.right);
+                if right.is_string() {
+                    Self::String
+                } else {
+                    Self::Undetermined
+                }
+            }
+            AssignmentOperator::Subtraction
+            | AssignmentOperator::Multiplication
+            | AssignmentOperator::Division
+            | AssignmentOperator::Remainder
+            | AssignmentOperator::ShiftLeft
+            | AssignmentOperator::BitwiseOR
+            | AssignmentOperator::ShiftRight
+            | AssignmentOperator::BitwiseXOR
+            | AssignmentOperator::BitwiseAnd
+            | AssignmentOperator::Exponential => {
+                let right = Self::from(&e.right);
+                if right.is_bigint() {
+                    Self::BigInt
+                } else if !(right.is_object() || right.is_undetermined()) {
+                    Self::Number
+                } else {
+                    Self::Undetermined
+                }
+            }
+            AssignmentOperator::ShiftRightZeroFill => Self::Number,
+            AssignmentOperator::LogicalAnd
+            | AssignmentOperator::LogicalOr
+            | AssignmentOperator::LogicalNullish => Self::Undetermined,
+        }
+    }
+}
+
 impl<'a> From<&ConditionalExpression<'a>> for ValueType {
     fn from(e: &ConditionalExpression<'a>) -> Self {
         let left = Self::from(&e.consequent);
diff --git a/crates/oxc_minifier/tests/ecmascript/value_type.rs b/crates/oxc_minifier/tests/ecmascript/value_type.rs
index d638e5b348efb..9fb5528637926 100644
--- a/crates/oxc_minifier/tests/ecmascript/value_type.rs
+++ b/crates/oxc_minifier/tests/ecmascript/value_type.rs
@@ -149,8 +149,32 @@ fn assignment_tests() {
     test("a = 1n", ValueType::BigInt);
     test("a = foo", ValueType::Undetermined);
 
-    // test("a += 1", ValueType::Undetermined);
-    // test("a += 1n", ValueType::Undetermined);
+    test("a += ''", ValueType::String);
+    // if `a` is a string, the result is string
+    // if `a` is an object, the result can be string or number or bigint
+    // if `a` is not a string nor an object, the result can be number or bigint
+    test("a += 1", ValueType::Undetermined);
+    test("a += 1n", ValueType::Undetermined);
+
+    test("a -= 1", ValueType::Number); // an error is thrown if `a` is converted to bigint
+    test("a -= undefined", ValueType::Number);
+    test("a -= null", ValueType::Number);
+    test("a -= ''", ValueType::Number);
+    test("a -= true", ValueType::Number);
+    test("a -= 1n", ValueType::BigInt); // an error is thrown if `a` is converted to number
+    test("a -= {}", ValueType::Undetermined); // number or bigint
+    test("a *= 1", ValueType::Number);
+    test("a /= 1", ValueType::Number);
+    test("a %= 1", ValueType::Number);
+    test("a <<= 1", ValueType::Number);
+    test("a |= 1", ValueType::Number);
+    test("a >>= 1", ValueType::Number);
+    test("a ^= 1", ValueType::Number);
+    test("a &= 1", ValueType::Number);
+    test("a **= 1", ValueType::Number);
+
+    test("a >>>= 1", ValueType::Number);
+    test("a >>>= 1n", ValueType::Number); // throws an error
 }
 
 #[test]