Add template functions to bind easily

[chokobole]

When we bind a function, we have to write argument checking and type conversion every time per each function. The example below is one of them.

#include "napi.h"

double CAdd(double arg0, double arg1) { return arg0 + arg1; }

Napi::Value Add(const Napi::CallbackInfo& info) {
  Napi::Env env = info.Env();
  if (info.Length() < 2) {
    Napi::TypeError::New(env, "Wrong number of arguments").ThrowAsJavaScriptException();
    return env.Null();
  }
  if (!info[0].IsNumber() || !info[1].IsNumber()) {
    Napi::TypeError::New(env, "Wrong arguments").ThrowAsJavaScriptException();
    return env.Null();
  }
  double arg0 = info[0].As<Napi::Number>().DoubleValue();
  double arg1 = info[1].As<Napi::Number>().DoubleValue();
  Napi::Number num = Napi::Number::New(env, CAdd(arg0, arg1);
  return num;
}

And I made some template functions not to write everytime like below.

#include "node_binding/typed_call.h"

double CAdd(double arg0, double arg1) { return arg0 + arg1; }

Napi::Value Add(const Napi::CallbackInfo& info) {
  return node_binding::TypedCall(info, &CAdd);
}

These codes are here and if you think it is good, I want to PR! :)

[gabrielschulhof]

@chokobole Thank you for working on this feature! We spoke about this in the past two N-API team meetings and we believe that the best course of action would be to release this as a package on npm so that we might later consider it for integration into node-addon-api. The reason we wish to take this approach is that node-addon-api's core mission is to expose the plain C N-API as C++ wrappers. This feature adds functionality on top of that core mission, and so might live as an independent package with a dependency on node-addon-api.

[legendecas]

Maybe effects can be made to help modules like this to incubate and expose to more audiences.

1. NPM integration of N-API headers like what node-addon-api do in https://github.com/nodejs/node-addon-api/blob/master/index.js;
2. links from node-addon-api documents;
3. NPM/GitHub tags.