How to return float32 array from c++ in node-addon-api?

Question

I am trying to run c++ in node.js using node-addon-api. I've read one well-written elaborate article from medium. But I want to pass to one of my functions a float, and a float array to another function. I want to return a float array from both of them. The example of a function (greeting.cpp)

#include <iostream>
#include <string>
#include <cmath>
#include "greeting.h"

float* helloUser( float name ) {
  float* result = new float[3];
  result[0] = 1;
  result[1] = 2;
  result[2] = name;
  return result;
}

And the code of index.cpp with a native implementation of the module:

#include <napi.h>
#include <string>
#include "greeting.h"

Napi::Float32Array greetHello(const Napi::CallbackInfo& info) {
    Napi::Env env = info.Env();

    float user = (float) info[0].ToNumber();
    float* result = helloUser( user );

    return Napi::Float32Array::New(env, result);
}

Napi::Object Init(Napi::Env env, Napi::Object exports) {

    exports.Set(
        Napi::String::New(env, "greetHello"), // property name => "greetHello"
        Napi::Function::New(env, greetHello) // property value => `greetHello` function
    );

    return exports;
}

NODE_API_MODULE(greet, Init)

The thing is, Napi::Float32Array doesn't work, I tried Napi::TypedArrayOf<float>, it doesn't work either. I am not that much into writing modules. I just want my pure c++ code running in node to send its result to the frontend. I tried accepting and returning floats with Napi::Number and it works fine. But I have no clue how to get it start working with arrays. Any help is appreciated.

Could you please expand on it, stark? You talk about function arguments, but I need to return from that function, too. I found a compromise solution for now - to pass and receive a string, and then parse it in JS to get my floats. But it apparently defeats the whole purpose of using c++. — agt-ru, May 19 '21 at 14:41
The resulting project resides here: https://github.com/agt-ru/cone3d I use c++ to make some calculations and return a string of float values, which are then parsed in node passed to the frontend. If anyone has suggestions for how to improve the performance and finally return and receive an array in c++/node communications, or may be provides a link to an article, where it is properly explained, I'll be really glad to hear the answer. — agt-ru, May 23 '21 at 02:52

score 0 · Answer 1 · answered Oct 27 '21 at 09:30

I ran into a similar issue and the Napi instance methods expect either a:

void MethodName(const Napi::CallbackInfo& info);

or

Napi::Value MethodName(const Napi::CallbackInfo& info);

Even though Napi::TypedArray inherits from Napi::Object which inherits from Napi::Value, it turns out you have to explicitly return a Napi::Value as follows:

Napi::Value greetHello(Napi::CallbackInfo const& info)
{
  Napi::Float32Array arr = Napi::Float32Array::New(info.Env(), 16);
  arr[0] = 1;
  arr[15] = 16;
  return arr;
}

Which will get you a Float32Array:

{
  greetHello: [Function (anonymous)]
}
Float32Array(16) [
  1, 0, 0,  0, 0, 0,
  0, 0, 0,  0, 0, 0,
  0, 0, 0, 16
]

How to return float32 array from c++ in node-addon-api?

1 Answers1