N_API How to send int value parameters to Napi::CallbackInfo - c++

Question

This My first node.js and n_api. I have been using PHP/APACHI. But I need the c++ library for my web And I decided to using n_api.
The problem is that the value sent by ajax is always 0 in c++.
I don't know what is problem.
ex) I using a vscode.
const testAddon = require('./build/Release/firstaddon.node');
var http = require('http');
var url = require('url');
var fs = require('fs');
const express = require('express');
const app = express();
bodyParser = require('body-parser');
var port = '1080';
app.use(bodyParser.json()); // to support JSON-encoded bodies
app.use(bodyParser.urlencoded({ // to support URL-encoded bodies
extended: true
}));
app.post('/server', function(req, res){
var responseData = {};
responseData.pID = req.body.pID;
console.log(responseData.pID); <<============= here, value is correct.
const prevInstance = new testAddon.ClassExample(4.3);
var value = prevInstance.getFile(responseData.pID);
console.log(value);
res.json(responseData);
});
If ajax sends 2, console.log(responseData.pID) //2 appears. It is normal.
Below is the classtest.cpp
Napi::Value ClassTest::GetFile(const Napi::CallbackInfo &info)
{
Napi::Env env = info.Env();
Napi::HandleScope scope(env);
using namespace std;
if (info.Length() != 1 || !info[0].IsNumber())
{
Napi::TypeError::New(env, "Number expected").ThrowAsJavaScriptException();
}
Napi::Number file_id = info[0].As<Napi::Number>();
int num = this->actualClass_->openFile(file_id); <<== here, file id
return Napi::Number::New(info.Env(), num);
}
And ActualClass.cpp Files showing problems.
int ActualClass::openFile(int id)
{
ifstream fin;
cout << id << endl; <<============================ here, always '0'
filename += to_string(id) += ".txt";
fin.open(filename.c_str(), ios_base::in | ios_base::binary);
if (fin.is_open())
{
while (fin.read((char *)&sdo, sizeof(sdo)))
{
cout << setw(20) << sdo.name << ":"
<< setprecision(0) << setw(12) << sdo.width
<< setprecision(2) << setw(6) << sdo.height
<< setprecision(4) << setw(6) << sdo.size << endl;
slist.push_back(sdo);
}
fin.close();
}
else if (!fin.is_open())
{
cerr << "can't open file " << filename << ".\n";
exit(EXIT_FAILURE);
}
return sdo.size;
}
Only files 1 to 4 are prepared.
But, the argument value entering the function is always 0.
Result is "can't open file 0.txt".
How can I solve it?
Napi::Number file_id = info[0].As<Napi::Number>();
I know here it is converted to an int value that can be handled by C++. Is there anything else I don't know?
Thanks for reading.

Answer 1

You need to cast it to the number with the help of the Napi::Number::Int32Value call. (You can also use Napi::Number::Int64Value for bigger numbers)
Try this.
int file_id = info[0].ToNumber().Int32Value();
Also unrelated to the question, but worth mentioning that when you are doing ThrowAsJavaScriptException() the actual C++ code keeps executing, you better return undefined to avoid nasty bugs.
if (info.Length() != 1 || !info[0].IsNumber())
{
Napi::TypeError::New(env, "Number expected").ThrowAsJavaScriptException();
return Env.Undefined();
}
A more clean way will be to enable the CPP exceptions and just throw the error in that place.
if (info.Length() != 1 || !info[0].IsNumber())
{
throw Napi::TypeError::New(env, "Number expected");
}