LibJS: Parse "if" statements

This patch implements basic parsing of "if" statements. We don't yet
support parsing "else", so I added a FIXME about that.

Libraries/LibJS/AST.cpp

@@ -91,8 +91,11 @@ Value IfStatement::execute(Interpreter& interpreter) const
 
     if (predicate_result.to_boolean())
         return interpreter.run(*m_consequent);
-    else
+
+    if (m_alternate)
         return interpreter.run(*m_alternate);
+
+    return js_undefined();
 }
 
 Value WhileStatement::execute(Interpreter& interpreter) const
@@ -427,9 +430,11 @@ void IfStatement::dump(int indent) const
     printf("If\n");
     predicate().dump(indent + 1);
     consequent().dump(indent + 1);
-    print_indent(indent);
+    if (alternate()) {
-    printf("Else\n");
+        print_indent(indent);
-    alternate().dump(indent + 1);
+        printf("Else\n");
+        alternate()->dump(indent + 1);
+    }
 }
 
 void WhileStatement::dump(int indent) const

Libraries/LibJS/AST.h

@@ -211,7 +211,7 @@ private:
 
 class IfStatement : public Statement {
 public:
-    IfStatement(NonnullRefPtr<Expression> predicate, NonnullRefPtr<ScopeNode> consequent, NonnullRefPtr<ScopeNode> alternate)
+    IfStatement(NonnullRefPtr<Expression> predicate, NonnullRefPtr<ScopeNode> consequent, RefPtr<ScopeNode> alternate)
         : m_predicate(move(predicate))
         , m_consequent(move(consequent))
         , m_alternate(move(alternate))
@@ -220,7 +220,7 @@ public:
 
     const Expression& predicate() const { return *m_predicate; }
     const ScopeNode& consequent() const { return *m_consequent; }
-    const ScopeNode& alternate() const { return *m_alternate; }
+    const ScopeNode* alternate() const { return m_alternate; }
 
     virtual Value execute(Interpreter&) const override;
     virtual void dump(int indent) const override;
@@ -230,7 +230,7 @@ private:
 
     NonnullRefPtr<Expression> m_predicate;
     NonnullRefPtr<ScopeNode> m_consequent;
-    NonnullRefPtr<ScopeNode> m_alternate;
+    RefPtr<ScopeNode> m_alternate;
 };
 
 class WhileStatement : public Statement {

Libraries/LibJS/Parser.cpp

@@ -194,6 +194,8 @@ NonnullRefPtr<Statement> Parser::parse_statement()
         return parse_variable_declaration();
     case TokenType::For:
         return parse_for_statement();
+    case TokenType::If:
+        return parse_if_statement();
     default:
         if (match_expression())
             return adopt(*new ExpressionStatement(parse_expression(0)));
@@ -512,6 +514,17 @@ NonnullRefPtr<VariableDeclaration> Parser::parse_variable_declaration()
     return create_ast_node<VariableDeclaration>(create_ast_node<Identifier>(name), move(initializer), declaration_type);
 }
 
+NonnullRefPtr<IfStatement> Parser::parse_if_statement()
+{
+    consume(TokenType::If);
+    consume(TokenType::ParenOpen);
+    auto predicate = parse_expression(0);
+    consume(TokenType::ParenClose);
+    auto consequent = parse_block_statement();
+    // FIXME: Parse "else"
+    return create_ast_node<IfStatement>(move(predicate), move(consequent), nullptr);
+}
+
 NonnullRefPtr<ForStatement> Parser::parse_for_statement()
 {
     consume(TokenType::For);

Libraries/LibJS/Parser.h

@@ -51,6 +51,7 @@ public:
     NonnullRefPtr<ReturnStatement> parse_return_statement();
     NonnullRefPtr<VariableDeclaration> parse_variable_declaration();
     NonnullRefPtr<ForStatement> parse_for_statement();
+    NonnullRefPtr<IfStatement> parse_if_statement();
 
     NonnullRefPtr<Expression> parse_expression(int min_precedence, Associativity associate = Associativity::Right);
     NonnullRefPtr<Expression> parse_primary_expression();