diff --git a/Libraries/LibJS/AST.cpp b/Libraries/LibJS/AST.cpp
index 1394c5dc05..436980ec55 100644
--- a/Libraries/LibJS/AST.cpp
+++ b/Libraries/LibJS/AST.cpp
@@ -383,6 +383,22 @@ Value AssignmentExpression::execute(Interpreter& interpreter) const
     case AssignmentOp::Assign:
         interpreter.set_variable(name, rhs_result);
         break;
+    case AssignmentOp::PlusEquals:
+        rhs_result = add(m_lhs->execute(interpreter), rhs_result);
+        interpreter.set_variable(name, rhs_result);
+        break;
+    case AssignmentOp::MinusEquals:
+        rhs_result = sub(m_lhs->execute(interpreter), rhs_result);
+        interpreter.set_variable(name, rhs_result);
+        break;
+    case AssignmentOp::AsteriskEquals:
+        rhs_result = mul(m_lhs->execute(interpreter), rhs_result);
+        interpreter.set_variable(name, rhs_result);
+        break;
+    case AssignmentOp::SlashEquals:
+        rhs_result = div(m_lhs->execute(interpreter), rhs_result);
+        interpreter.set_variable(name, rhs_result);
+        break;
     }
     return rhs_result;
 }
@@ -394,6 +410,18 @@ void AssignmentExpression::dump(int indent) const
     case AssignmentOp::Assign:
         op_string = "=";
         break;
+    case AssignmentOp::PlusEquals:
+        op_string = "+=";
+        break;
+    case AssignmentOp::MinusEquals:
+        op_string = "-=";
+        break;
+    case AssignmentOp::AsteriskEquals:
+        op_string = "*=";
+        break;
+    case AssignmentOp::SlashEquals:
+        op_string = "/=";
+        break;
     }
 
     ASTNode::dump(indent);
diff --git a/Libraries/LibJS/AST.h b/Libraries/LibJS/AST.h
index decfa5137b..6c3f11b8f1 100644
--- a/Libraries/LibJS/AST.h
+++ b/Libraries/LibJS/AST.h
@@ -391,6 +391,10 @@ private:
 
 enum class AssignmentOp {
     Assign,
+    PlusEquals,
+    MinusEquals,
+    AsteriskEquals,
+    SlashEquals,
 };
 
 class AssignmentExpression : public Expression {
diff --git a/Libraries/LibJS/Parser.cpp b/Libraries/LibJS/Parser.cpp
index 38e278300c..c022b96b6c 100644
--- a/Libraries/LibJS/Parser.cpp
+++ b/Libraries/LibJS/Parser.cpp
@@ -124,15 +124,27 @@ NonnullOwnPtr<Expression> Parser::parse_secondary_expression(NonnullOwnPtr<Expre
     case TokenType::Plus:
         consume();
         return make<BinaryExpression>(BinaryOp::Plus, move(lhs), parse_expression());
+    case TokenType::PlusEquals:
+        consume();
+        return make<AssignmentExpression>(AssignmentOp::PlusEquals, move(lhs), parse_expression());
     case TokenType::Minus:
         consume();
         return make<BinaryExpression>(BinaryOp::Minus, move(lhs), parse_expression());
+    case TokenType::MinusEquals:
+        consume();
+        return make<AssignmentExpression>(AssignmentOp::MinusEquals, move(lhs), parse_expression());
     case TokenType::Asterisk:
         consume();
         return make<BinaryExpression>(BinaryOp::Asterisk, move(lhs), parse_expression());
+    case TokenType::AsteriskEquals:
+        consume();
+        return make<AssignmentExpression>(AssignmentOp::AsteriskEquals, move(lhs), parse_expression());
     case TokenType::Slash:
         consume();
         return make<BinaryExpression>(BinaryOp::Slash, move(lhs), parse_expression());
+    case TokenType::SlashEquals:
+        consume();
+        return make<AssignmentExpression>(AssignmentOp::SlashEquals, move(lhs), parse_expression());
     case TokenType::GreaterThanEquals:
         consume();
         return make<BinaryExpression>(BinaryOp::GreaterThanEquals, move(lhs), parse_expression());
@@ -250,9 +262,13 @@ bool Parser::match_secondary_expression() const
 {
     auto type = m_current_token.type();
     return type == TokenType::Plus
+        || type == TokenType::PlusEquals
         || type == TokenType::Minus
+        || type == TokenType::MinusEquals
         || type == TokenType::Asterisk
+        || type == TokenType::AsteriskEquals
         || type == TokenType::Slash
+        || type == TokenType::SlashEquals
         || type == TokenType::Equals
         || type == TokenType::GreaterThanEquals
         || type == TokenType::LessThanEquals