Adding two string arguments in Node Addons

I am trying to add two string arguments in my addon but I could not get it to work. I always get an error about the types of the two arguments.

My function:

void Add(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  // Check the number of arguments passed.
  if (args.Length() < 2) {
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate, "Wrong number of arguments")
            .ToLocalChecked()));
    return;
  }

  // Check the argument types
  if (!args[0]->IsString() || !args[1]->IsString()) {
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate, "Wrong arguments").ToLocalChecked()));
    return;
  }

  Local<String> arg1 = args[0]->ToString(context).ToLocalChecked();
  Local<String> arg2 = args[1]->ToString(context).ToLocalChecked();

  args.GetReturnValue().Set(arg1 + arg2);
}

The Whole File:

#include <node.h>
#include <string>

namespace demo {

using v8::Context;
using v8::Exception;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

void Add(const FunctionCallbackInfo<Value> &args) {...

void Init(Local<Object> exports) { NODE_SET_METHOD(exports, "add", Add); }

NODE_MODULE(NODE_GYP_MODULE_NAME, Init);

}

Any feedback would be appreciated.

Solution

Found the solution. The only problem was that I should have converted the Local<String> type to std::string. And after adding them together, convert them back to Local<String> when returning the value.

Code:

void Add(const FunctionCallbackInfo<Value> &args) {
  Isolate *isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  // Check the number of arguments passed.
  if (args.Length() < 2) {
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate, "Wrong number of arguments")
            .ToLocalChecked()));
    return;
  }

  // Check the argument types
  if (!args[0]->IsString() || !args[1]->IsString()) {
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate, "Wrong arguments").ToLocalChecked()));
    return;
  }

  v8::String::Utf8Value str1(isolate, args[0]);
  v8::String::Utf8Value str2(isolate, args[1]);
  std::string searchWord(*str1);
  std::string body(*str2);

  std::string res = searchWord + body;

  args.GetReturnValue().Set(
      String::NewFromUtf8(isolate, res.c_str()).ToLocalChecked());
}