Calling JavaScript from C++ with node.js

Question

Is there a way to call JS functions from C++ through node.js (as callbacks or something like that)? If yes, how? I'm searching for it on the web, but haven't found any helpful resource.

Thanks in advance

Answer 1

the answer is yes, and here is a way to do it :

I have created a js file called index.js and here is the path to the file

D:\NodeJS\call_js_from_cpp 

it contains the following code

console.log("hello");

Now I have a C++ code that executes the following shell command and stores the output in the variable "output" (which is a string):

node D:\NodeJS\call_js_from_cpp\index.js

Here is the C++ code : (note that in the code, there are double \\ in the path so it becomes like this node D:\\NodeJS\\call_js_from_cpp\\index.js)

#include<iostream>
#include<fstream>
#include<string>
#include<cstdlib>
#include<sstream>

std::string ssystem (const char *command) {
  char tmpname [L_tmpnam];
  std::tmpnam ( tmpname );
  std::string scommand = command;
  std::string cmd = scommand + " >> " + tmpname;
  std::system(cmd.c_str());
  std::ifstream file(tmpname, std::ios::in );
  std::string result;
  if (file) {
   while (!file.eof()) result.push_back(file.get());
   file.close();
  }
  remove(tmpname);
  return result;
}

//for cygwin

int main(int argc, char *argv[])
{
   std::string bash = "node D:\\NodeJS\\call_js_from_cpp\\index.js";
   std::string in;
   std::string s = ssystem(bash.c_str());
   std::istringstream iss(s);
   std::string output;
   while ( std::getline(iss, output) )
    {
      std::cout << output;
    }

   return 0;
 }

Answer 2

One way to do it form a native addon can be using the provided function as a callback, for example let's gonna assume that you have a function named setPrintFunction() declared in your native environment (A native addon):

(Call this for example main.cc )

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

v8::Persistent<v8::Function> fn;

// Call this at any time, but after the capture!
void printToNode(std::string msg) {
  auto isolate = fn->GetIsolate();
  // This part is the one that transforms your std::string to a javascript
  // string, and passes it as the first argument:
  const unsigned argc = 1;
  auto argv[argc] = {
      v8::String::NewFromUtf8(isolate,
                          msg.c_str(),
                          v8::NewStringType::kNormal).ToLocalChecked()
  };
  cb->Call(context, Null(isolate), argc, argv).ToLocalChecked();
}

// This is your native function that captures the reference
void setPrintFunction(const v8::FunctionCallbackInfo<Value>& args) {
  auto isolate = args.GetIsolate();
  auto context = isolate->GetCurrentContext();
  auto cb = v8::Local<v8::Function>::Cast(args[0]);
  fn = v8::Persistent<v8::Function>::New(cb);
}

// This part exports the function
void Init(v8::Local<v8::Object> exports, v8::Local<v8::Object> module) {
  NODE_SET_METHOD(module, "exports", setPrintFunction);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

Then, just importing your addon and using it like:

(Call this for example index.js )

const { setPrintFunction } = require('<your path to .node file>');

function printNodeMsg(msg) {
  console.log('<msg>: ' + msg);
}

setPrintFunction(printNodeMsg);

So what you're basically doing is capturing the reference to the v8::Function (Which is the javascript function, but in the native environment) and then invoking it and passing "Hello World!" as the first (and unique) parameter.

More on the subject: https://nodejs.org/api/addons.html

Answer 3

Of course you can. For example, if you want to write a simple factorial function in C++ , you could do something like

#include <node.h>

using namespace v8;

int factorial(int n) {
    if (n == 0) return 1;
    else return n * factorial(n - 1);
}

void Factorial(const FunctionCallbackInfo<Value>& args) {
    Isolate* isolate = Isolate::GetCurrent();
    HandleScope scope(isolate);

    if (args.Length() != 2) {
        isolate->ThrowException(Exception::TypeError(String::NewFromUtf8(isolate, "Wrong number of arguments")));
    } else {
        if (!(args[0]->IsNumber() && args[1]->IsFunction())) {
            isolate->ThrowException(Exception::TypeError(String::NewFromUtf8(isolate, "Wrong arguments type")));
        } else {
            int result = factorial(args[0]->Int32Value());

            Local<Function> callbackFunction = Local<Function>::Cast(args[1]);
            const unsigned argc = 1;
            Local<Value> argv[argc] = { Number::New(isolate, result) };

            callbackFunction->Call(isolate->GetCurrentContext()->Global(), argc, argv);
        }
    }
}

void Init(Handle<Object> exports) {
    NODE_SET_METHOD(exports, "factorial", Factorial);
}

NODE_MODULE(Factorial, Init)

And in your JavaScript file, call it like this

var factorialAddon = require('./addons/Factorial');
factorialAddon.factorial(5, function (result) {
    console.log(result);
});