diff --git a/jerry-core/ecma/base/ecma-helpers-string.c b/jerry-core/ecma/base/ecma-helpers-string.c
index a773fcd145..d2c46c57d9 100644
--- a/jerry-core/ecma/base/ecma-helpers-string.c
+++ b/jerry-core/ecma/base/ecma-helpers-string.c
@@ -395,10 +395,8 @@ ecma_concat_ecma_strings (ecma_string_t *string1_p, /**< first ecma-string */
     string_desc_p->refs_and_container = ECMA_STRING_CONTAINER_HEAP_ASCII_STRING | ECMA_STRING_REF_ONE;
     const size_t data_size = new_size;
     lit_utf8_byte_t *data_p = (lit_utf8_byte_t *) jmem_heap_alloc_block (data_size);
-    lit_utf8_size_t bytes_copied = ecma_string_to_utf8_string (string1_p, data_p, str1_size);
-    JERRY_ASSERT (bytes_copied == str1_size);
-    bytes_copied = ecma_string_to_utf8_string (string2_p, data_p + str1_size, str2_size);
-    JERRY_ASSERT (bytes_copied == str2_size);
+    ecma_string_to_utf8_bytes (string1_p, data_p, str1_size);
+    ecma_string_to_utf8_bytes (string2_p, data_p + str1_size, str2_size);
 
     string_desc_p->u.ascii_string.size = (uint16_t) new_size;
 
@@ -411,15 +409,14 @@ ecma_concat_ecma_strings (ecma_string_t *string1_p, /**< first ecma-string */
     string_desc_p->refs_and_container = ECMA_STRING_CONTAINER_HEAP_UTF8_STRING | ECMA_STRING_REF_ONE;
     const size_t data_size = new_size + sizeof (ecma_string_heap_header_t);
     ecma_string_heap_header_t *data_p = (ecma_string_heap_header_t *) jmem_heap_alloc_block (data_size);
-    lit_utf8_size_t bytes_copied = ecma_string_to_utf8_string (string1_p,
-                                                               (lit_utf8_byte_t *) (data_p + 1),
-                                                               str1_size);
-    JERRY_ASSERT (bytes_copied == str1_size);
-
-    bytes_copied = ecma_string_to_utf8_string (string2_p,
-                                               (lit_utf8_byte_t *) (data_p + 1) + str1_size,
-                                               str2_size);
-    JERRY_ASSERT (bytes_copied == str2_size);
+    ecma_string_to_utf8_bytes (string1_p,
+                               (lit_utf8_byte_t *) (data_p + 1),
+                               str1_size);
+
+    ecma_string_to_utf8_bytes (string2_p,
+                               (lit_utf8_byte_t *) (data_p + 1) + str1_size,
+                               str2_size);
+
     JERRY_ASSERT (string_length <= UINT16_MAX);
 
     data_p->size = (uint16_t) new_size;
@@ -675,10 +672,10 @@ ecma_string_get_array_index (const ecma_string_t *str_p, /**< ecma-string */
  * @return number of bytes, actually copied to the buffer.
  */
 lit_utf8_size_t __attr_return_value_should_be_checked___
-ecma_string_to_utf8_string (const ecma_string_t *string_desc_p, /**< ecma-string descriptor */
-                            lit_utf8_byte_t *buffer_p, /**< destination buffer pointer
-                                                        * (can be NULL if buffer_size == 0) */
-                            lit_utf8_size_t buffer_size) /**< size of buffer */
+ecma_string_copy_to_utf8_buffer (const ecma_string_t *string_desc_p, /**< ecma-string descriptor */
+                                 lit_utf8_byte_t *buffer_p, /**< destination buffer pointer
+                                                             * (can be NULL if buffer_size == 0) */
+                                 lit_utf8_size_t buffer_size) /**< size of buffer */
 {
   JERRY_ASSERT (string_desc_p != NULL);
   JERRY_ASSERT (string_desc_p->refs_and_container >= ECMA_STRING_REF_ONE);
@@ -741,7 +738,22 @@ ecma_string_to_utf8_string (const ecma_string_t *string_desc_p, /**< ecma-string
 
   JERRY_ASSERT (size <= buffer_size);
   return size;
-} /* ecma_string_to_utf8_string */
+} /* ecma_string_copy_to_utf8_buffer */
+
+/**
+ * Convert ecma-string's contents to a cesu-8 string and put it to the buffer.
+ * It is the caller's responsibility to make sure that the string fits in the buffer.
+ * Check if the size of the string is equal with the size of the buffer.
+ */
+void __attr_always_inline___
+ecma_string_to_utf8_bytes (const ecma_string_t *string_desc_p, /**< ecma-string descriptor */
+                           lit_utf8_byte_t *buffer_p, /**< destination buffer pointer
+                                                       * (can be NULL if buffer_size == 0) */
+                           lit_utf8_size_t buffer_size) /**< size of buffer */
+{
+  const lit_utf8_size_t size = ecma_string_copy_to_utf8_buffer (string_desc_p, buffer_p, buffer_size);
+  JERRY_ASSERT (size == buffer_size);
+} /* ecma_string_to_utf8_bytes */
 
 /**
  * Lengths for numeric string values
@@ -1018,8 +1030,7 @@ ecma_compare_ecma_strings_longpath (const ecma_string_t *string1_p, /* ecma-stri
   {
     utf8_string1_p = (lit_utf8_byte_t *) jmem_heap_alloc_block ((size_t) strings_size);
 
-    lit_utf8_size_t bytes_copied = ecma_string_to_utf8_string (string1_p, utf8_string1_p, strings_size);
-    JERRY_ASSERT (bytes_copied == strings_size);
+    ecma_string_to_utf8_bytes (string1_p, utf8_string1_p, strings_size);
 
     is_utf8_string1_on_heap = true;
   }
@@ -1049,11 +1060,11 @@ ecma_compare_ecma_strings_longpath (const ecma_string_t *string1_p, /* ecma-stri
   {
     utf8_string2_p = (lit_utf8_byte_t *) jmem_heap_alloc_block ((size_t) strings_size);
 
-    lit_utf8_size_t bytes_copied = ecma_string_to_utf8_string (string2_p, utf8_string2_p, strings_size);
-    JERRY_ASSERT (bytes_copied == strings_size);
+    ecma_string_to_utf8_bytes (string2_p, utf8_string2_p, strings_size);
 
     is_utf8_string2_on_heap = true;
   }
+
   const bool is_equal = !strncmp ((char *) utf8_string1_p, (char *) utf8_string2_p, (size_t) strings_size);
 
   if (is_utf8_string1_on_heap)
@@ -1160,8 +1171,7 @@ ecma_compare_ecma_strings_relational (const ecma_string_t *string1_p, /**< ecma-
       utf8_string1_p = utf8_string1_buffer;
     }
 
-    lit_utf8_size_t bytes_copied = ecma_string_to_utf8_string (string1_p, utf8_string1_p, utf8_string1_size);
-    JERRY_ASSERT (bytes_copied == utf8_string1_size);
+    ecma_string_to_utf8_bytes (string1_p, utf8_string1_p, utf8_string1_size);
   }
 
   if (ECMA_STRING_GET_CONTAINER (string2_p) == ECMA_STRING_CONTAINER_HEAP_UTF8_STRING)
@@ -1202,8 +1212,7 @@ ecma_compare_ecma_strings_relational (const ecma_string_t *string1_p, /**< ecma-
       utf8_string2_p = utf8_string2_buffer;
     }
 
-    lit_utf8_size_t bytes_copied = ecma_string_to_utf8_string (string2_p, utf8_string2_p, utf8_string2_size);
-    JERRY_ASSERT (bytes_copied == utf8_string2_size);
+    ecma_string_to_utf8_bytes (string2_p, utf8_string2_p, utf8_string2_size);
   }
 
   bool is_first_less_than_second = lit_compare_utf8_strings_relational (utf8_string1_p,
@@ -1345,8 +1354,7 @@ ecma_string_get_char_at_pos (const ecma_string_t *string_p, /**< ecma-string */
 
   JMEM_DEFINE_LOCAL_ARRAY (utf8_str_p, buffer_size, lit_utf8_byte_t);
 
-  lit_utf8_size_t sz = ecma_string_to_utf8_string (string_p, utf8_str_p, buffer_size);
-  JERRY_ASSERT (sz == buffer_size);
+  ecma_string_to_utf8_bytes (string_p, utf8_str_p, buffer_size);
 
   JERRY_ASSERT (ECMA_STRING_GET_CONTAINER (string_p) == ECMA_STRING_CONTAINER_UINT32_IN_DESC);
   /* Both above must be ascii strings. */
@@ -1504,8 +1512,7 @@ ecma_string_substr (const ecma_string_t *string_p, /**< pointer to an ecma strin
 
     JMEM_DEFINE_LOCAL_ARRAY (utf8_str_p, buffer_size, lit_utf8_byte_t);
 
-    lit_utf8_size_t sz = ecma_string_to_utf8_string (string_p, utf8_str_p, buffer_size);
-    JERRY_ASSERT (sz == buffer_size);
+    ecma_string_to_utf8_bytes (string_p, utf8_str_p, buffer_size);
 
     /**
      * II. Extract substring
diff --git a/jerry-core/ecma/base/ecma-helpers.h b/jerry-core/ecma/base/ecma-helpers.h
index e9354a4c37..8b40fc0739 100644
--- a/jerry-core/ecma/base/ecma-helpers.h
+++ b/jerry-core/ecma/base/ecma-helpers.h
@@ -66,8 +66,7 @@
   if (utf8_ptr == NULL) \
   { \
     utf8_ptr = (const lit_utf8_byte_t *) jmem_heap_alloc_block (utf8_str_size); \
-    lit_utf8_size_t sz = ecma_string_to_utf8_string (ecma_str_ptr, (lit_utf8_byte_t *) utf8_ptr, utf8_str_size); \
-    JERRY_ASSERT (sz == utf8_str_size); \
+    ecma_string_to_utf8_bytes (ecma_str_ptr, (lit_utf8_byte_t *) utf8_ptr, utf8_str_size); \
     utf8_ptr ## must_be_freed = true; \
   }
 
@@ -177,7 +176,8 @@ extern ecma_number_t ecma_string_to_number (const ecma_string_t *);
 extern bool ecma_string_get_array_index (const ecma_string_t *, uint32_t *);
 
 extern lit_utf8_size_t __attr_return_value_should_be_checked___
-ecma_string_to_utf8_string (const ecma_string_t *, lit_utf8_byte_t *, lit_utf8_size_t);
+ecma_string_copy_to_utf8_buffer (const ecma_string_t *, lit_utf8_byte_t *, lit_utf8_size_t);
+extern void ecma_string_to_utf8_bytes (const ecma_string_t *, lit_utf8_byte_t *, lit_utf8_size_t);
 extern const lit_utf8_byte_t *ecma_string_raw_chars (const ecma_string_t *, lit_utf8_size_t *, bool *);
 
 extern bool ecma_compare_ecma_strings_equal_hashes (const ecma_string_t *, const ecma_string_t *);
diff --git a/jerry-core/ecma/builtin-objects/ecma-builtin-error-prototype.c b/jerry-core/ecma/builtin-objects/ecma-builtin-error-prototype.c
index a3a64328b8..df43813d4a 100644
--- a/jerry-core/ecma/builtin-objects/ecma-builtin-error-prototype.c
+++ b/jerry-core/ecma/builtin-objects/ecma-builtin-error-prototype.c
@@ -142,7 +142,7 @@ ecma_builtin_error_prototype_object_to_string (ecma_value_t this_arg) /**< this
           JMEM_DEFINE_LOCAL_ARRAY (ret_str_buffer, size, lit_utf8_byte_t);
           lit_utf8_byte_t *ret_str_buffer_p = ret_str_buffer;
 
-          lit_utf8_size_t bytes = ecma_string_to_utf8_string (name_string_p, ret_str_buffer_p, name_size);
+          lit_utf8_size_t bytes = ecma_string_copy_to_utf8_buffer (name_string_p, ret_str_buffer_p, name_size);
           JERRY_ASSERT (bytes == name_size);
           ret_str_buffer_p = ret_str_buffer_p + bytes;
           JERRY_ASSERT (ret_str_buffer_p <= ret_str_buffer + size);
@@ -157,7 +157,7 @@ ecma_builtin_error_prototype_object_to_string (ecma_value_t this_arg) /**< this
                                                               space_size);
           JERRY_ASSERT (ret_str_buffer_p <= ret_str_buffer + size);
 
-          bytes = ecma_string_to_utf8_string (msg_string_p, ret_str_buffer_p, msg_size);
+          bytes = ecma_string_copy_to_utf8_buffer (msg_string_p, ret_str_buffer_p, msg_size);
           JERRY_ASSERT (bytes == msg_size);
           ret_str_buffer_p = ret_str_buffer_p + bytes;
           JERRY_ASSERT (ret_str_buffer_p == ret_str_buffer + size);
diff --git a/jerry-core/ecma/builtin-objects/ecma-builtin-global.c b/jerry-core/ecma/builtin-objects/ecma-builtin-global.c
index 5c86026b37..60e3eca226 100644
--- a/jerry-core/ecma/builtin-objects/ecma-builtin-global.c
+++ b/jerry-core/ecma/builtin-objects/ecma-builtin-global.c
@@ -85,8 +85,7 @@ ecma_builtin_global_object_print (ecma_value_t this_arg __attr_unused___, /**< t
                              utf8_str_size,
                              lit_utf8_byte_t);
 
-    lit_utf8_size_t actual_sz = ecma_string_to_utf8_string (str_p, utf8_str_p, utf8_str_size);
-    JERRY_ASSERT (actual_sz == utf8_str_size);
+    ecma_string_to_utf8_bytes (str_p, utf8_str_p, utf8_str_size);
 
     const lit_utf8_byte_t *utf8_str_curr_p = utf8_str_p;
     const lit_utf8_byte_t *utf8_str_end_p = utf8_str_p + utf8_str_size;
@@ -739,10 +738,7 @@ ecma_builtin_global_object_decode_uri_helper (ecma_value_t uri __attr_unused___,
                            input_size + 1,
                            lit_utf8_byte_t);
 
-  lit_utf8_size_t sz = ecma_string_to_utf8_string (input_string_p,
-                                                   input_start_p,
-                                                   input_size);
-  JERRY_ASSERT (sz == input_size);
+  ecma_string_to_utf8_bytes (input_string_p, input_start_p, input_size);
 
   input_start_p[input_size] = LIT_BYTE_NULL;
 
@@ -1017,10 +1013,7 @@ ecma_builtin_global_object_encode_uri_helper (ecma_value_t uri, /**< uri argumen
                            input_size,
                            lit_utf8_byte_t);
 
-  lit_utf8_size_t sz = ecma_string_to_utf8_string (input_string_p,
-                                                   input_start_p,
-                                                   input_size);
-  JERRY_ASSERT (sz == input_size);
+  ecma_string_to_utf8_bytes (input_string_p, input_start_p, input_size);
 
   /*
    * The URI encoding has two major phases: first we validate the input,
@@ -1238,10 +1231,7 @@ ecma_builtin_global_object_escape (ecma_value_t this_arg __attr_unused___, /**<
                            input_size,
                            lit_utf8_byte_t);
 
-  lit_utf8_size_t sz = ecma_string_to_utf8_string (input_string_p,
-                                                   input_start_p,
-                                                   input_size);
-  JERRY_ASSERT (sz == input_size);
+  ecma_string_to_utf8_bytes (input_string_p, input_start_p, input_size);
 
   /*
    * The escape routine has two major phases: first we compute
@@ -1358,8 +1348,7 @@ ecma_builtin_global_object_unescape (ecma_value_t this_arg __attr_unused___, /**
 
   /* 3. */
   JMEM_DEFINE_LOCAL_ARRAY (input_start_p, input_size, lit_utf8_byte_t);
-  lit_utf8_size_t sz = ecma_string_to_utf8_string (input_string_p, input_start_p, input_size);
-  JERRY_ASSERT (sz == input_size);
+  ecma_string_to_utf8_bytes (input_string_p, input_start_p, input_size);
 
   const lit_utf8_byte_t *input_curr_p = input_start_p;
   const lit_utf8_byte_t *input_end_p = input_start_p + input_size;
diff --git a/jerry-core/ecma/builtin-objects/ecma-builtin-json.c b/jerry-core/ecma/builtin-objects/ecma-builtin-json.c
index ae3ac51886..4239e2acad 100644
--- a/jerry-core/ecma/builtin-objects/ecma-builtin-json.c
+++ b/jerry-core/ecma/builtin-objects/ecma-builtin-json.c
@@ -712,7 +712,7 @@ ecma_builtin_json_parse (ecma_value_t this_arg __attr_unused___, /**< 'this' arg
 
   JMEM_DEFINE_LOCAL_ARRAY (str_start_p, buffer_size, lit_utf8_byte_t);
 
-  const lit_utf8_size_t sz = ecma_string_to_utf8_string (string_p, str_start_p, buffer_size);
+  const lit_utf8_size_t sz = ecma_string_copy_to_utf8_buffer (string_p, str_start_p, buffer_size);
   JERRY_ASSERT (sz == string_size);
 
   str_start_p[string_size] = LIT_BYTE_NULL;
diff --git a/jerry-core/jerry.c b/jerry-core/jerry.c
index 8c8f83d551..500d0ba55b 100644
--- a/jerry-core/jerry.c
+++ b/jerry-core/jerry.c
@@ -540,7 +540,7 @@ jerry_string_to_char_buffer (const jerry_string_t *string_p, /**< string descrip
 {
   jerry_assert_api_available ();
 
-  return ecma_string_to_utf8_string (string_p, (lit_utf8_byte_t *) buffer_p, buffer_size);
+  return ecma_string_copy_to_utf8_buffer (string_p, (lit_utf8_byte_t *) buffer_p, buffer_size);
 } /* jerry_string_to_char_buffer */
 
 /**