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A first project

Before you start, make sure you’ve got all the necessary prerequisites and tools installed.

As you select where to begin, you should be aware that N-API operates at two levels which we can think of as the “C level” and the “C++ level”.

The “C level” code is built entirely into Node itself and is very well documented on the Node documentation pages. If you need low-level access to the intricacies of Node, this is the tool for you.

Alternatively, there is the node-addon-api package which adds a C++ wrapper to the N-API code built into Node. This package makes working with N-API much easier as it implements a very nice object model and abstracts away much of the detailed coding that would otherwise be required, while retaining the N-API promise of ABI stability and forward compatibility.

This tutorial uses node-addon-api.

N-API has been in public release and active development starting with Node 8.0.0. Since then, it’s undergone a number of refinements. This tutorial has been tested with Node 10.10.0 and is known to fail with older versions of Node. You will need a copy of Node that supports N-API in order to develop and run N-API code. To see which versions of Node support N-API, refer to the N-API Version Matrix. You can determine the version of Node you’re running with the command node -v.

Creating a project

The easiest way to create a new N-API project is to use the generator-napi-module package. As the package documentation describes, generator-napi-module relies on Yeoman which must also be installed:

npm install -g yo
npm install -g generator-napi-module

On some systems, you may receive the error Error: EACCES: permission denied, access. In that case, on Mac and Linux systems you need to run the commands with elevated privileges:

sudo npm install -g yo
sudo npm install -g generator-napi-module

Using nvm is an excellent way to banish permission issues.

Then enter these commands to generate a new project:

mkdir hello-world
cd hello-world
yo napi-module

Here are the prompts you’ll see and some suggested responses:

package name: (hello-world) 
version: (1.0.0) 
description: A first project.
git repository: 
keywords: 
author: Your name goes here
license: (ISC) 

Yeoman will display the generated package.json file here.

Is this OK? (yes) yes
? Choose a template Hello World
? Would you like to generate TypeScript wrappers for your module? No

Yeoman will now build your “Hello World” add-on module.

At this point, you might try running npm test to make sure everything is correctly installed:

npm test

Project structure

At this point you have a completely functional N-API module project. The project files are structured according to N-API best practices. It should look like this:

.
├── binding.gyp                         Used by gyp to compile the C code
├── build                               The intermediary and final build products
│   └── < contents not shown here >
├── lib                                 The N-API code that accesses the C/C++ binary
│   └── binding.js
├── node_modules                        Node modules required by your project
│   └── < contents not shown here >
├── package.json                        npm description of your module
├── package-lock.json                   Used by npm to insure deployment consistency
├── src                                 The C/C++ code
│   └── hello_world.cc
└── test                                Test code
    └── test_binding.js

Let’s take a look at the essential files.

package.json

package.json

{
  "main": "lib/binding.js",
  "private": true,
  "dependencies": {
    "node-addon-api": "^1.1.0"
  },
  "scripts": {
    "test": "node --napi-modules ./test/test_binding.js"
  },
  "gypfile": true,
  "name": "hello-world",
  "version": "1.0.0",
  "description": "A first project.",
  "author": "Your name goes here",
  "license": "ISC"
}

This is a typical package.json file as generated by Yeoman from the responses we entered earlier to the yo napi-module command. There are a couple of entries of interest here.

Notice the node-addon-api dependency. This package, which is not strictly a part of Node, adds a C++ wrapper to the C API implemented in Node. The package makes it very straightforward to create and manipulate JavaScript objects inside C++. The package is useful even if the underlying library you’re accessing is in C.

There is also a "gypfile": true entry which informs npm that your package requires a build using the capabilities of the node-gyp package which is covered next.

binding.gyp

binding.gyp

{
  'targets': [
    {
      'target_name': 'hello-world-native',
      'sources': [ 'src/hello_world.cc' ],
      'include_dirs': ["<!@(node -p \"require('node-addon-api').include\")"],
      'dependencies': ["<!(node -p \"require('node-addon-api').gyp\")"],
      'cflags!': [ '-fno-exceptions' ],
      'cflags_cc!': [ '-fno-exceptions' ],
      'xcode_settings': {
        'GCC_ENABLE_CPP_EXCEPTIONS': 'YES',
        'CLANG_CXX_LIBRARY': 'libc++',
        'MACOSX_DEPLOYMENT_TARGET': '10.7'
      },
      'msvs_settings': {
        'VCCLCompilerTool': { 'ExceptionHandling': 1 },
      }
    }
  ]
}

One of the challenges of making C/C++ code available to Node is getting the code compiled, linked, and packaged for a variety of operating systems and architectures. Historically, this would require learning the intricacies of a variety of build tools across a number of operating systems. This is the specific issue GYP seeks to address.

Using GYP permits having a single configuration file that works across all platforms and architectures GYP supports. (It’s GYP, by the way, that requires Python).

node-gyp is a command line tool built in Node that orchestrates GYP to compile your C/C++ files to the correct destination. When npm sees the "gypfile": true entry in your package.json file, it automatically invokes its own internal copy of node-gyp which looks for this binding.gyp file which must be called binding.gyp in order for node-gyp to locate it.

The binding.gyp file is a GYP file which is thoroughly documented here. There is also specific information about building libraries here.

src/hello_world.cc

hello_world.cc

#include <napi.h>

using namespace Napi;

Napi::String Method(const Napi::CallbackInfo& info) {
  Napi::Env env = info.Env();
  return Napi::String::New(env, "world");
}

Napi::Object Init(Napi::Env env, Napi::Object exports) {
  exports.Set(Napi::String::New(env, "HelloWorld"),
              Napi::Function::New(env, Method));
  return exports;
}

NODE_API_MODULE(addon, Init)

This is perhaps the simplest useful(?) N-API file you can have.

The file defines a C++ Method function that takes a single Napi::CallbackInfo& argument. This info argument is used to access the JavaScript environment, including any JavaScript arguments that might be passed in.

info is an array of JavaScript arguments.

In this case, the C++ Method function uses the info argument to create a Napi::Env local that is then used to create a Napi::String object which is returned with the value “world”.

The C++ Init function shows how to set a single export value for the native add-on module. In this case the name of the export is “HelloWorld” and the value is the Method function.

The NODE_API_MODULE macro at the bottom of the C++ file insures that the Init function is called when the module is loaded.

lib/binding.js

binding.js

const addon = require('../build/Release/hello-world-native');

module.exports = addon.HelloWorld

This JavaScript file defines a JavaScript class that acts as an intermediary to the C++ binary.

In this case, the sole export of the binding is the HelloWorld function.

test/test_binding.js

test_binding.js

const HelloWorld = require("../lib/binding.js");
const assert = require("assert");

assert(HelloWorld, "The expected function is undefined");

function testBasic()
{
    const result =  HelloWorld("hello");
    assert.strictEqual(result, "world", "Unexpected value returned");
}

assert.doesNotThrow(testBasic, undefined, "testBasic threw an expection");

console.log("Tests passed- everything looks OK!");

This code demonstrates how to load and call the HelloWorld function using JavaScript. Recall that the sole export from the binding is the HelloWorld function. The function is loaded into the HelloWorld variable using the require command.

The testBasic function then calls the HelloWorld function and verifies the result.

Conclusion

This project demonstrates a very simple N-API module that exports a single function. In addition, here are some things you might want to try:

  • Run test_binding.js in your debugger. See if you can step through the code to get a better understanding of how it works. What sort of visibility are you getting into the JavaScript object created by the C++ code?
  • Modify test_binding.js to use the C++ binary module directly instead of through binding.js. Step through the code in your debugger to see how things are different.
  • Modify hello_world.cc to access arguments passed from the JavaScript. Hint: The node-addon-api module comes with examples.

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