LibJS: Implement typeof operator

Base/home/anon/js/typeof.js

@@ -0,0 +1 @@
+console.log(typeof undefined, typeof true, typeof 'a', typeof 1, typeof {});

Libraries/LibJS/AST.cpp

@@ -199,6 +199,23 @@ Value UnaryExpression::execute(Interpreter& interpreter) const
         return bitwise_not(lhs_result);
     case UnaryOp::Not:
         return Value(!lhs_result.to_boolean());
+    case UnaryOp::Typeof:
+        switch (lhs_result.type()) {
+        case Value::Type::Undefined:
+            return js_string(interpreter.heap(), "undefined");
+        case Value::Type::Null:
+            // yes, this is on purpose. yes, this is how javascript works.
+            // yes, it's silly.
+            return js_string(interpreter.heap(), "object");
+        case Value::Type::Number:
+            return js_string(interpreter.heap(), "number");
+        case Value::Type::String:
+            return js_string(interpreter.heap(), "string");
+        case Value::Type::Object:
+            return js_string(interpreter.heap(), "object");
+        case Value::Type::Boolean:
+            return js_string(interpreter.heap(), "boolean");
+        }
     }
 
     ASSERT_NOT_REACHED();
@@ -318,6 +335,9 @@ void UnaryExpression::dump(int indent) const
     case UnaryOp::Not:
         op_string = "!";
         break;
+    case UnaryOp::Typeof:
+        op_string = "typeof ";
+        break;
     }
 
     print_indent(indent);

Libraries/LibJS/AST.h

@@ -309,6 +309,7 @@ private:
 enum class UnaryOp {
     BitwiseNot,
     Not,
+    Typeof,
 };
 
 class UnaryExpression : public Expression {

Libraries/LibJS/Parser.cpp

@@ -254,6 +254,9 @@ NonnullOwnPtr<Expression> Parser::parse_unary_prefixed_expression()
     case TokenType::Tilde:
         consume();
         return make<UnaryExpression>(UnaryOp::BitwiseNot, parse_primary_expression());
+    case TokenType::Typeof:
+        consume();
+        return make<UnaryExpression>(UnaryOp::Typeof, parse_primary_expression());
     default:
         m_has_errors = true;
         expected("primary expression (missing switch case)");
@@ -532,7 +535,8 @@ bool Parser::match_unary_prefixed_expression() const
     return type == TokenType::PlusPlus
         || type == TokenType::MinusMinus
         || type == TokenType::ExclamationMark
-        || type == TokenType::Tilde;
+        || type == TokenType::Tilde
+        || type == TokenType::Typeof;
 }
 
 bool Parser::match_secondary_expression() const