v8::String::Concat must not throw.

src/api.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #ifdef V8_USE_ADDRESS_SANITIZER
 #include <sanitizer/asan_interface.h>
 #endif  // V8_USE_ADDRESS_SANITIZER
@@ skipping 5532 matching lines @@
                    length);
 }
 
 
 Local<String> v8::String::Concat(Handle<String> left, Handle<String> right) {
   i::Handle<i::String> left_string = Utils::OpenHandle(*left);
   i::Isolate* isolate = left_string->GetIsolate();
   LOG_API(isolate, "String::New(char)");
   ENTER_V8(isolate);
   i::Handle<i::String> right_string = Utils::OpenHandle(*right);
-  // We do not expect this to fail. Change this if it does.
+  // If we are steering towards a range error, do not wait for the error to be
+  // thrown, and return the null handle instead.
+  if (left_string->length() + right_string->length() > i::String::kMaxLength) {
+    return Local<String>();
+  }
   i::Handle<i::String> result = isolate->factory()->NewConsString(
       left_string, right_string).ToHandleChecked();
   return Utils::ToLocal(result);
 }
 
 
 static i::MaybeHandle<i::String> NewExternalStringHandle(
     i::Isolate* isolate, v8::String::ExternalStringResource* resource) {
   return isolate->factory()->NewExternalStringFromTwoByte(resource);
 }
@@ skipping 2219 matching lines @@
   Isolate* isolate = reinterpret_cast<Isolate*>(info.GetIsolate());
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 } }  // namespace v8::internal