diff --git a/src/iotjs.cpp b/src/iotjs.cpp
index 6d8fc2e8f7..c7c6983548 100644
--- a/src/iotjs.cpp
+++ b/src/iotjs.cpp
@@ -1,4 +1,4 @@
-/* Copyright 2015 Samsung Electronics Co., Ltd.
+/* Copyright 2015-2016 Samsung Electronics Co., Ltd.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -91,7 +91,7 @@ static void CleanupModules() {
 static bool RunIoTjs(JObject* process) {
   // Evaluating 'iotjs.js' returns a function.
 #ifndef ENABLE_SNAPSHOT
-  JResult jmain = JObject::Eval(String(iotjs_s), false, false);
+  JResult jmain = JObject::Eval(String(iotjs_s, iotjs_l), false, false);
 #else
   JResult jmain = JObject::ExecSnapshot(iotjs_s, iotjs_l);
 #endif
diff --git a/src/iotjs_binding.cpp b/src/iotjs_binding.cpp
index 5179e343f0..d63c152bac 100644
--- a/src/iotjs_binding.cpp
+++ b/src/iotjs_binding.cpp
@@ -1,4 +1,4 @@
-/* Copyright 2015 Samsung Electronics Co., Ltd.
+/* Copyright 2015-2016 Samsung Electronics Co., Ltd.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -105,10 +105,9 @@ JObject::JObject(const char* v) {
 
 
 JObject::JObject(const String& v) {
-  IOTJS_ASSERT(!v.IsEmpty());
   _obj_val.type = JERRY_API_DATA_TYPE_STRING;
-  _obj_val.u.v_string = jerry_api_create_string(
-      reinterpret_cast<const jerry_api_char_t*>(v.data()));
+  _obj_val.u.v_string = jerry_api_create_string_sz(
+      reinterpret_cast<const jerry_api_char_t*>(v.data()), v.size());
   _unref_at_close = true;
 }
 
@@ -170,71 +169,36 @@ JObject JObject::Error(const char* message) {
 }
 
 
-JObject JObject::Error(const String& message) {
-  return CreateError(message.data(), JERRY_API_ERROR_COMMON);
-}
-
-
 JObject JObject::EvalError(const char* message) {
   return CreateError(message, JERRY_API_ERROR_EVAL);
 }
 
 
-JObject JObject::EvalError(const String& message) {
-  return CreateError(message.data(), JERRY_API_ERROR_EVAL);
-}
-
-
 JObject JObject::RangeError(const char* message) {
   return CreateError(message, JERRY_API_ERROR_RANGE);
 }
 
 
-JObject JObject::RangeError(const String& message) {
-  return CreateError(message.data(), JERRY_API_ERROR_RANGE);
-}
-
-
 JObject JObject::ReferenceError(const char* message) {
   return CreateError(message, JERRY_API_ERROR_REFERENCE);
 }
 
 
-JObject JObject::ReferenceError(const String& message) {
-  return CreateError(message.data(), JERRY_API_ERROR_REFERENCE);
-}
-
-
 JObject JObject::SyntaxError(const char* message) {
   return CreateError(message, JERRY_API_ERROR_SYNTAX);
 }
 
 
-JObject JObject::SyntaxError(const String& message) {
-  return CreateError(message.data(), JERRY_API_ERROR_SYNTAX);
-}
-
-
 JObject JObject::TypeError(const char* message) {
   return CreateError(message, JERRY_API_ERROR_TYPE);
 }
 
 
-JObject JObject::TypeError(const String& message) {
-  return CreateError(message.data(), JERRY_API_ERROR_TYPE);
-}
-
-
 JObject JObject::URIError(const char* message) {
   return CreateError(message, JERRY_API_ERROR_URI);
 }
 
 
-JObject JObject::URIError(const String& message) {
-  return CreateError(message.data(), JERRY_API_ERROR_URI);
-}
-
-
 JResult JObject::Eval(const String& source,
                       bool direct_mode,
                       bool strict_mode) {
@@ -275,11 +239,6 @@ void JObject::SetProperty(const char* name, const JObject& val) {
 }
 
 
-void JObject::SetProperty(const String& name, const JObject& val) {
-  SetProperty(name.data(), val);
-}
-
-
 void JObject::SetProperty(const char* name, JRawValueType val) {
   IOTJS_ASSERT(IsObject());
   bool is_ok  = jerry_api_set_object_field_value(
@@ -290,11 +249,6 @@ void JObject::SetProperty(const char* name, JRawValueType val) {
 }
 
 
-void JObject::SetProperty(const String& name, JRawValueType val) {
-  SetProperty(name.data(), val);
-}
-
-
 JObject JObject::GetProperty(const char* name) {
   IOTJS_ASSERT(IsObject());
   JRawValueType res;
@@ -307,11 +261,6 @@ JObject JObject::GetProperty(const char* name) {
 }
 
 
-JObject JObject::GetProperty(const String& name) {
-  return GetProperty(name.data());
-}
-
-
 void JObject::Ref() {
   if (JVAL_IS_STRING(&_obj_val)) {
     jerry_api_acquire_string(_obj_val.u.v_string);
@@ -446,7 +395,7 @@ String JObject::GetString() {
 
   jerry_api_size_t size = jerry_api_get_string_size(_obj_val.u.v_string);
 
-  String res("", size);
+  String res(NULL, size);
 
   jerry_api_char_t* buffer = reinterpret_cast<jerry_api_char_t*>(res.data());
 
diff --git a/src/iotjs_binding.h b/src/iotjs_binding.h
index 9f729294ab..58f745b87e 100644
--- a/src/iotjs_binding.h
+++ b/src/iotjs_binding.h
@@ -1,4 +1,4 @@
-/* Copyright 2015 Samsung Electronics Co., Ltd.
+/* Copyright 2015-2016 Samsung Electronics Co., Ltd.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -91,26 +91,12 @@ class JObject {
 
   // Create a javascript error object.
   static JObject Error(const char* message);
-  static JObject Error(const String& message);
-
   static JObject EvalError(const char* message);
-  static JObject EvalError(const String& message);
-
   static JObject RangeError(const char* message);
-  static JObject RangeError(const String& message);
-
   static JObject ReferenceError(const char* message);
-  static JObject ReferenceError(const String& message);
-
   static JObject SyntaxError(const char* message);
-  static JObject SyntaxError(const String& message);
-
   static JObject TypeError(const char* message);
-  static JObject TypeError(const String& message);
-
   static JObject URIError(const char* message);
-  static JObject URIError(const String& message);
-
 
   // Evaluate javascript source file.
   static JResult Eval(const String& source,
@@ -143,13 +129,9 @@ class JObject {
 
   // Sets & gets property for the javascript object.
   void SetProperty(const char* name, const JObject& val);
-  void SetProperty(const String& name, const JObject& val);
-
   void SetProperty(const char* name, JRawValueType val);
-  void SetProperty(const String& name, JRawValueType val);
 
   JObject GetProperty(const char* name);
-  JObject GetProperty(const String& name);
 
   // Sets & gets native data for the javascript object.
   void SetNative(uintptr_t ptr, JFreeHandlerType free_handler);
diff --git a/src/iotjs_module_buffer.cpp b/src/iotjs_module_buffer.cpp
index 4ac895281a..85ac0aec65 100644
--- a/src/iotjs_module_buffer.cpp
+++ b/src/iotjs_module_buffer.cpp
@@ -1,4 +1,4 @@
-/* Copyright 2015 Samsung Electronics Co., Ltd.
+/* Copyright 2015-2016 Samsung Electronics Co., Ltd.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -268,9 +268,9 @@ JHANDLER_FUNCTION(ToString) {
   int length = end - start;
   JHANDLER_CHECK(length >= 0);
 
-  String str("", length + 1);
+  length = strnlen(buffer_wrap->buffer() + start, length);
 
-  strncpy(str.data(), buffer_wrap->buffer() + start, length);
+  String str(buffer_wrap->buffer() + start, length);
 
   JObject ret(str);
   handler.Return(ret);
diff --git a/src/iotjs_module_httpparser.cpp b/src/iotjs_module_httpparser.cpp
index d63ed0480c..4177dccf03 100644
--- a/src/iotjs_module_httpparser.cpp
+++ b/src/iotjs_module_httpparser.cpp
@@ -1,4 +1,4 @@
-/* Copyright 2015 Samsung Electronics Co., Ltd.
+/* Copyright 2015-2016 Samsung Electronics Co., Ltd.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -116,8 +116,8 @@ class HTTPParserWrap : public JObjectWrap {
       char index_string1 [5];
       sprintf(index_string0,"%d",i*2);
       sprintf(index_string1,"%d",i*2+1);
-      JObject v(values[i].data());
-      JObject f(fields[i].data());
+      JObject v(values[i]);
+      JObject f(fields[i]);
       jheader.SetProperty(index_string0, f);
       jheader.SetProperty(index_string1, v);
 
@@ -147,7 +147,7 @@ class HTTPParserWrap : public JObjectWrap {
       JSETPROPERTY(info, "headers", makeHeader());
       if ( parser.type == HTTP_REQUEST) {
         IOTJS_ASSERT(!url.IsEmpty());
-        JSETPROPERTY(info, "url", url.data());
+        JSETPROPERTY(info, "url", url);
       }
     }
     n_fields = n_values = 0;
@@ -160,7 +160,7 @@ class HTTPParserWrap : public JObjectWrap {
     // Status
     if (parser.type == HTTP_RESPONSE) {
       JSETPROPERTY(info, "status", (int32_t)parser.status_code);
-      JSETPROPERTY(info, "status_msg", status_msg.data());
+      JSETPROPERTY(info, "status_msg", status_msg);
     }
 
 
@@ -218,7 +218,7 @@ class HTTPParserWrap : public JObjectWrap {
     JObject jheader(makeHeader());
     argv.Add(jheader);
     if (parser.type == HTTP_REQUEST && !url.IsEmpty()) {
-      JObject jurl(url.data());
+      JObject jurl(url);
       argv.Add(jurl);
     }
 
diff --git a/src/iotjs_module_process.cpp b/src/iotjs_module_process.cpp
index d51407aded..960a591ab4 100644
--- a/src/iotjs_module_process.cpp
+++ b/src/iotjs_module_process.cpp
@@ -1,4 +1,4 @@
-/* Copyright 2015 Samsung Electronics Co., Ltd.
+/* Copyright 2015-2016 Samsung Electronics Co., Ltd.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -121,19 +121,18 @@ JHANDLER_FUNCTION(Binding) {
 }
 
 
-static JResult WrapEval(const char* source) {
+static JResult WrapEval(const char* source, size_t length) {
   static const char* wrapper[2] = {
       "(function(exports, require, module) {\n",
       "});\n" };
 
-  int len1 = strlen(wrapper[0]);
-  int len2 = strlen(source);
-  int len3 = strlen(wrapper[1]);
+  int len0 = strlen(wrapper[0]);
+  int len1 = strlen(wrapper[1]);
 
-  String code(NULL, len1 + len2 + len3);
+  String code(NULL, len0 + length + len1);
   strcpy(code.data(), wrapper[0]);
-  strcpy(code.data() + len1, source);
-  strcpy(code.data() + len1 + len2, wrapper[1]);
+  strcpy(code.data() + len0, source);
+  strcpy(code.data() + len0 + length, wrapper[1]);
 
   return JObject::Eval(code);
 }
@@ -143,7 +142,9 @@ JHANDLER_FUNCTION(Compile){
   JHANDLER_CHECK(handler.GetArgLength() == 1);
   JHANDLER_CHECK(handler.GetArg(0)->IsString());
 
-  JResult jres = WrapEval(handler.GetArg(0)->GetString().data());
+  String source = handler.GetArg(0)->GetString();
+
+  JResult jres = WrapEval(source.data(), source.size());
 
   if (jres.IsOk()) {
     handler.Return(jres.value());
@@ -159,20 +160,11 @@ JHANDLER_FUNCTION(CompileNativePtr){
   JHANDLER_CHECK(handler.GetArgLength() == 1);
   JHANDLER_CHECK(handler.GetArg(0)->IsString());
 
-  String id (handler.GetArg(0)->GetString());
+  String id = handler.GetArg(0)->GetString();
 
   int i=0;
   while (natives[i].name != NULL) {
-    const char *name_iter_p = natives[i].name;
-    size_t name_len = 0;
-    while (*name_iter_p != '\0')
-    {
-      name_len++;
-      name_iter_p++;
-    }
-
-    if (name_len == (size_t) id.size ()
-        && !strncmp (natives[i].name, id.data(), id.size())) {
+    if (!strcmp(natives[i].name, id.data())) {
       break;
     }
 
@@ -184,7 +176,7 @@ JHANDLER_FUNCTION(CompileNativePtr){
     JResult jres = JObject::ExecSnapshot(natives[i].code,
                                          natives[i].length);
 #else
-    JResult jres = WrapEval((const char*)natives[i].code);
+    JResult jres = WrapEval((const char*)natives[i].code, natives[i].length);
 #endif
 
     if (jres.IsOk()) {
diff --git a/src/iotjs_util.cpp b/src/iotjs_util.cpp
index e59215e25b..df003df016 100644
--- a/src/iotjs_util.cpp
+++ b/src/iotjs_util.cpp
@@ -1,4 +1,4 @@
-/* Copyright 2015 Samsung Electronics Co., Ltd.
+/* Copyright 2015-2016 Samsung Electronics Co., Ltd.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -33,18 +33,12 @@ String ReadFile(const char* path) {
   IOTJS_ASSERT(len >= 0);
   fseek(file, 0, SEEK_SET);
 
-  String contents(NULL, 0, len);
+  String contents(NULL, len);
 
-  char buff[128];
-  size_t total = 0;
+  size_t read = fread(contents.data(), 1, len, file);
+  IOTJS_ASSERT(read == len);
 
-  while (total < len) {
-    size_t read = fread(buff, 1, 128, file);
-    IOTJS_ASSERT(read > 0);
-
-    contents.Append(buff, read);
-    total += read;
-  }
+  *(contents.data() + len) = 0;
 
   fclose(file);
 
diff --git a/src/iotjs_util.h b/src/iotjs_util.h
index 6dc91d2385..1ccd3ad622 100644
--- a/src/iotjs_util.h
+++ b/src/iotjs_util.h
@@ -1,4 +1,4 @@
-/* Copyright 2015 Samsung Electronics Co., Ltd.
+/* Copyright 2015-2016 Samsung Electronics Co., Ltd.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -51,6 +51,9 @@ class String {
   explicit String(const char* data, int size = -1, int cap = -1);
 
   // Create string object from other string object.
+  // (Actually unimplemented. Declaration of copy constructor is needed for
+  // return value optimization to work. However, linker error will be emitted
+  // if copy construction is used in any other situation.)
   String(const String& other);
 
   // Destructor