LibJS: Implement modulo assignment operator (%=)

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/72d2bd56ce5
Pull-request: https://github.com/SerenityOS/serenity/pull/2105

Libraries/LibJS/AST.cpp

@@ -850,6 +850,12 @@ Value AssignmentExpression::execute(Interpreter& interpreter) const
             return {};
         rhs_result = div(interpreter, lhs_result, rhs_result);
         break;
+    case AssignmentOp::ModuloAssignment:
+        lhs_result = m_lhs->execute(interpreter);
+        if (interpreter.exception())
+            return {};
+        rhs_result = mod(interpreter, lhs_result, rhs_result);
+        break;
     case AssignmentOp::ExponentiationAssignment:
         lhs_result = m_lhs->execute(interpreter);
         if (interpreter.exception())
@@ -960,6 +966,9 @@ void AssignmentExpression::dump(int indent) const
     case AssignmentOp::DivisionAssignment:
         op_string = "/=";
         break;
+    case AssignmentOp::ModuloAssignment:
+        op_string = "%=";
+        break;
     case AssignmentOp::ExponentiationAssignment:
         op_string = "**=";
         break;

Libraries/LibJS/AST.h

@@ -604,6 +604,7 @@ enum class AssignmentOp {
     SubtractionAssignment,
     MultiplicationAssignment,
     DivisionAssignment,
+    ModuloAssignment,
     ExponentiationAssignment,
     BitwiseAndAssignment,
     BitwiseOrAssignment,

Libraries/LibJS/Parser.cpp

@@ -634,6 +634,9 @@ NonnullRefPtr<Expression> Parser::parse_secondary_expression(NonnullRefPtr<Expre
     case TokenType::Percent:
         consume();
         return create_ast_node<BinaryExpression>(BinaryOp::Modulo, move(lhs), parse_expression(min_precedence, associativity));
+    case TokenType::PercentEquals:
+        consume();
+        return create_ast_node<AssignmentExpression>(AssignmentOp::ModuloAssignment, move(lhs), parse_expression(min_precedence, associativity));
     case TokenType::DoubleAsterisk:
         consume();
         return create_ast_node<BinaryExpression>(BinaryOp::Exponentiation, move(lhs), parse_expression(min_precedence, associativity));
@@ -1194,6 +1197,7 @@ bool Parser::match_secondary_expression() const
         || type == TokenType::Slash
         || type == TokenType::SlashEquals
         || type == TokenType::Percent
+        || type == TokenType::PercentEquals
         || type == TokenType::DoubleAsterisk
         || type == TokenType::DoubleAsteriskEquals
         || type == TokenType::Equals

Libraries/LibJS/Tests/assignment-operators.js

@@ -23,6 +23,10 @@ try {
     assert((x /= 2) === 3);
     assert(x === 3);
 
+    x = 6;
+    assert((x %= 4) === 2);
+    assert(x === 2);
+
     x = 2;
     assert((x **= 3) === 8);
     assert(x === 8);