LibJS: Implement basic support for the "new" keyword

NewExpression mostly piggybacks on the existing CallExpression. The big
difference is that "new" creates a new Object and passes it as |this|
to the callee.

Libraries/LibJS/AST.cpp

@@ -80,18 +80,30 @@ Value CallExpression::execute(Interpreter& interpreter) const
             return {};
     }
 
-    if (m_callee->is_member_expression()) {
-        auto object_value = static_cast<const MemberExpression&>(*m_callee).object().execute(interpreter);
-        if (interpreter.exception())
-            return {};
-        auto this_value = object_value.to_object(interpreter.heap());
-        if (interpreter.exception())
-            return {};
-        call_frame.this_value = this_value;
+    Object* new_object = nullptr;
+    if (is_new_expression()) {
+        new_object = interpreter.heap().allocate<Object>();
+        call_frame.this_value = new_object;
+    } else {
+        if (m_callee->is_member_expression()) {
+            auto object_value = static_cast<const MemberExpression&>(*m_callee).object().execute(interpreter);
+            if (interpreter.exception())
+                return {};
+            auto this_value = object_value.to_object(interpreter.heap());
+            if (interpreter.exception())
+                return {};
+            call_frame.this_value = this_value;
+        }
     }
 
     auto result = function->call(interpreter, call_frame.arguments);
     interpreter.pop_call_frame();
+
+    if (is_new_expression()) {
+        if (result.is_object())
+            return result;
+        return new_object;
+    }
     return result;
 }
 

Libraries/LibJS/AST.h

@@ -54,6 +54,7 @@ public:
     virtual bool is_member_expression() const { return false; }
     virtual bool is_scope_node() const { return false; }
     virtual bool is_variable_declaration() const { return false; }
+    virtual bool is_new_expression() const { return false; }
 
 protected:
     ASTNode() {}
@@ -470,7 +471,7 @@ private:
 
 class CallExpression : public Expression {
 public:
-    explicit CallExpression(NonnullRefPtr<Expression> callee, NonnullRefPtrVector<Expression> arguments = {})
+    CallExpression(NonnullRefPtr<Expression> callee, NonnullRefPtrVector<Expression> arguments = {})
         : m_callee(move(callee))
         , m_arguments(move(arguments))
     {
@@ -486,6 +487,18 @@ private:
     const NonnullRefPtrVector<Expression> m_arguments;
 };
 
+class NewExpression final : public CallExpression {
+public:
+    NewExpression(NonnullRefPtr<Expression> callee, NonnullRefPtrVector<Expression> arguments = {})
+        : CallExpression(move(callee), move(arguments))
+    {
+    }
+
+private:
+    virtual const char * class_name() const override { return "NewExpression"; }
+    virtual bool is_new_expression() const override { return true; }
+};
+
 enum class AssignmentOp {
     Assignment,
     AdditionAssignment,

Libraries/LibJS/Parser.cpp

@@ -247,6 +247,8 @@ NonnullRefPtr<Expression> Parser::parse_primary_expression()
         return parse_function_node<FunctionExpression>();
     case TokenType::BracketOpen:
         return parse_array_expression();
+    case TokenType::New:
+        return parse_new_expression();
     default:
         m_has_errors = true;
         expected("primary expression (missing switch case)");
@@ -443,6 +445,29 @@ NonnullRefPtr<CallExpression> Parser::parse_call_expression(NonnullRefPtr<Expres
     return create_ast_node<CallExpression>(move(lhs), move(arguments));
 }
 
+NonnullRefPtr<NewExpression> Parser::parse_new_expression()
+{
+    consume(TokenType::New);
+
+    // FIXME: Support full expressions as the callee as well.
+    auto callee = create_ast_node<Identifier>(consume(TokenType::Identifier).value());
+
+    NonnullRefPtrVector<Expression> arguments;
+
+    if (match(TokenType::ParenOpen)) {
+        consume(TokenType::ParenOpen);
+        while (match_expression()) {
+            arguments.append(parse_expression(0));
+            if (!match(TokenType::Comma))
+                break;
+            consume();
+        }
+        consume(TokenType::ParenClose);
+    }
+
+    return create_ast_node<NewExpression>(move(callee), move(arguments));
+}
+
 NonnullRefPtr<ReturnStatement> Parser::parse_return_statement()
 {
     consume(TokenType::Return);

Libraries/LibJS/Parser.h

@@ -63,6 +63,7 @@ public:
     NonnullRefPtr<ArrayExpression> parse_array_expression();
     NonnullRefPtr<Expression> parse_secondary_expression(NonnullRefPtr<Expression>, int min_precedence, Associativity associate = Associativity::Right);
     NonnullRefPtr<CallExpression> parse_call_expression(NonnullRefPtr<Expression>);
+    NonnullRefPtr<NewExpression> parse_new_expression();
 
     bool has_errors() const { return m_has_errors; }
 

Libraries/LibJS/Tests/constructor-basic.js

@@ -0,0 +1,7 @@
+function Foo() {
+    this.x = 123;
+}
+
+var foo = new Foo();
+if (foo.x === 123)
+    console.log("PASS");