# Addons#

Addons are dynamically linked shared objects. They can provide glue to C and C++ libraries. The API (at the moment) is rather complex, involving knowledge of several libraries:

- V8 JavaScript, a C++ library. Used for interfacing with JavaScript: creating objects, calling functions, etc. Documented mostly in the `v8.h` header file (`deps/v8/include/v8.h` in the Node source tree), which is also available online.

- libuv, C event loop library. Anytime one needs to wait for a file descriptor to become readable, wait for a timer, or wait for a signal to be received one will need to interface with libuv. That is, if you perform any I/O, libuv will need to be used.

- Internal Node libraries. Most importantly is the `node::ObjectWrap` class which you will likely want to derive from.

- Others. Look in `deps/` for what else is available.

Node statically compiles all its dependencies into the executable. When compiling your module, you don't need to worry about linking to any of these libraries.

All of the following examples are available for download and may be used as a starting-point for your own Addon.

## Hello world#

To get started let's make a small Addon which is the C++ equivalent of the following JavaScript code:

    module.exports.hello = function() { return 'world'; };

First we create a file `hello.cc`:

    // hello.cc
    #include <node.h>

    using namespace v8;

    void Method(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = Isolate::GetCurrent();
      HandleScope scope(isolate);
      args.GetReturnValue().Set(String::NewFromUtf8(isolate, "world"));
    }

    void init(Handle<Object> exports) {
      NODE_SET_METHOD(exports, "hello", Method);
    }

    NODE_MODULE(addon, init)

Note that all Node addons must export an initialization function:

    void Initialize (Handle<Object> exports);
    NODE_MODULE(module_name, Initialize)

There is no semi-colon after `NODE_MODULE` as it's not a function (see `node.h`).

The `module_name` needs to match the filename of the final binary (minus the .node suffix).

The source code needs to be built into `addon.node`, the binary Addon. To do this we create a file called `binding.gyp` which describes the configuration to build your module in a JSON-like format. This file gets compiled by node-gyp.

    {
      "targets": [
        {
          "target_name": "addon",
          "sources": [ "hello.cc" ]
        }
      ]
    }

The next step is to generate the appropriate project build files for the current platform. Use `node-gyp configure` for that.

Now you will have either a `Makefile` (on Unix platforms) or a `vcxproj` file (on Windows) in the `build/` directory. Next invoke the `node-gyp build` command.

Now you have your compiled `.node` bindings file! The compiled bindings end up in `build/Release/`.

You can now use the binary addon in a Node project `hello.js` by pointing `require` to the recently built `hello.node` module:

    // hello.js
    var addon = require('./build/Release/addon');

    console.log(addon.hello()); // 'world'

Please see patterns below for further information or

https://github.com/arturadib/node-qt for an example in production.

## Addon patterns#

Below are some addon patterns to help you get started. Consult the online v8 reference for help with the various v8 calls, and v8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc.

In order to use these examples you need to compile them using `node-gyp`. Create the following `binding.gyp` file:

    {
      "targets": [
        {
          "target_name": "addon",
          "sources": [ "addon.cc" ]
        }
      ]
    }

In cases where there is more than one `.cc` file, simply add the file name to the `sources` array, e.g.:

    "sources": ["addon.cc", "myexample.cc"]

Now that you have your `binding.gyp` ready, you can configure and build the addon:

    $ node-gyp configure build

### Function arguments#

The following pattern illustrates how to read arguments from JavaScript function calls and return a result. This is the main and only needed source `addon.cc`:

    // addon.cc
    #include <node.h>

    using namespace v8;

    void Add(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")));
        return;
      }

      if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
        isolate->ThrowException(Exception::TypeError(
            String::NewFromUtf8(isolate, "Wrong arguments")));
        return;
      }

      double value = args[0]->NumberValue() + args[1]->NumberValue();
      Local<Number> num = Number::New(isolate, value);

      args.GetReturnValue().Set(num);
    }

    void Init(Handle<Object> exports) {
      NODE_SET_METHOD(exports, "add", Add);
    }

    NODE_MODULE(addon, Init)

You can test it with the following JavaScript snippet:

    // test.js
    var addon = require('./build/Release/addon');

    console.log( 'This should be eight:', addon.add(3,5) );

### Callbacks#

You can pass JavaScript functions to a C++ function and execute them from there. Here's `addon.cc`:

    // addon.cc
    #include <node.h>

    using namespace v8;

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

      Local<Function> cb = Local<Function>::Cast(args[0]);
      const unsigned argc = 1;
      Local<Value> argv[argc] = { String::NewFromUtf8(isolate, "hello world") };
      cb->Call(isolate->GetCurrentContext()->Global(), argc, argv);
    }

    void Init(Handle<Object> exports, Handle<Object> module) {
      NODE_SET_METHOD(module, "exports", RunCallback);
    }

    NODE_MODULE(addon, Init)

Note that this example uses a two-argument form of `Init()` that receives the full `module` object as the second argument. This allows the addon to completely overwrite `exports` with a single function instead of adding the function as a property of `exports`.

To test it run the following JavaScript snippet:

    // test.js
    var addon = require('./build/Release/addon');

    addon(function(msg){
      console.log(msg); // 'hello world'
    });

### Object factory#

You can create and return new objects from within a C++ function with this `addon.cc` pattern, which returns an object with property `msg` that echoes the string passed to `createObject()`:

    // addon.cc
    #include <node.h>

    using namespace v8;

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

      Local<Object> obj = Object::New(isolate);
      obj->Set(String::NewFromUtf8(isolate, "msg"), args[0]->ToString());

      args.GetReturnValue().Set(obj);
    }

    void Init(Handle<Object> exports, Handle<Object> module) {
      NODE_SET_METHOD(module, "exports", CreateObject);
    }

    NODE_MODULE(addon, Init)

To test it in JavaScript:

    // test.js
    var addon = require('./build/Release/addon');

    var obj1 = addon('hello');
    var obj2 = addon('world');
    console.log(obj1.msg+' '+obj2.msg); // 'hello world'

### Function factory#

This pattern illustrates how to create and return a JavaScript function that wraps a C++ function:

    // addon.cc
    #include <node.h>

    using namespace v8;

    void MyFunction(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = Isolate::GetCurrent();
      HandleScope scope(isolate);
      args.GetReturnValue().Set(String::NewFromUtf8(isolate, "hello world"));
    }

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

      Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction);
      Local<Function> fn = tpl->GetFunction();

      // omit this to make it anonymous
      fn->SetName(String::NewFromUtf8(isolate, "theFunction"));

      args.GetReturnValue().Set(fn);
    }

    void Init(Handle<Object> exports, Handle<Object> module) {
      NODE_SET_METHOD(module, "exports", CreateFunction);
    }

    NODE_MODULE(addon, Init)

To test:

    // test.js
    var addon = require('./build/Release/addon');

    var fn = addon();
    console.log(fn()); // 'hello world'

### Wrapping C++ objects#

Here we will create a wrapper for a C++ object/class `MyObject` that can be instantiated in JavaScript through the `new` operator. First prepare the main module `addon.cc`:

    // addon.cc
    #include <node.h>
    #include "myobject.h"

    using namespace v8;

    void InitAll(Handle<Object> exports) {
      MyObject::Init(exports);
    }

    NODE_MODULE(addon, InitAll)

Then in `myobject.h` make your wrapper inherit from `node::ObjectWrap`:

    // myobject.h
    #ifndef MYOBJECT_H
    #define MYOBJECT_H

    #include <node.h>
    #include <node_object_wrap.h>

    class MyObject : public node::ObjectWrap {
     public:
      static void Init(v8::Handle<v8::Object> exports);

     private:
      explicit MyObject(double value = 0);
      ~MyObject();

      static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
      static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
      static v8::Persistent<v8::Function> constructor;
      double value_;
    };

    #endif

And in `myobject.cc` implement the various methods that you want to expose. Here we expose the method `plusOne` by adding it to the constructor's prototype:

    // myobject.cc
    #include "myobject.h"

    using namespace v8;

    Persistent<Function> MyObject::constructor;

    MyObject::MyObject(double value) : value_(value) {
    }

    MyObject::~MyObject() {
    }

    void MyObject::Init(Handle<Object> exports) {
      Isolate* isolate = Isolate::GetCurrent();

      // Prepare constructor template
      Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
      tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
      tpl->InstanceTemplate()->SetInternalFieldCount(1);

      // Prototype
      NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

      constructor.Reset(isolate, tpl->GetFunction());
      exports->Set(String::NewFromUtf8(isolate, "MyObject"),
                   tpl->GetFunction());
    }

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

      if (args.IsConstructCall()) {
        // Invoked as constructor: `new MyObject(...)`
        double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
        MyObject* obj = new MyObject(value);
        obj->Wrap(args.This());
        args.GetReturnValue().Set(args.This());
      } else {
        // Invoked as plain function `MyObject(...)`, turn into construct call.
        const int argc = 1;
        Local<Value> argv[argc] = { args[0] };
        Local<Function> cons = Local<Function>::New(isolate, constructor);
        args.GetReturnValue().Set(cons->NewInstance(argc, argv));
      }
    }

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

      MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
      obj->value_ += 1;

      args.GetReturnValue().Set(Number::New(isolate, obj->value_));
    }

Test it with:

    // test.js
    var addon = require('./build/Release/addon');

    var obj = new addon.MyObject(10);
    console.log( obj.plusOne() ); // 11
    console.log( obj.plusOne() ); // 12
    console.log( obj.plusOne() ); // 13

### Factory of wrapped objects#

This is useful when you want to be able to create native objects without explicitly instantiating them with the `new` operator in JavaScript, e.g.

    var obj = addon.createObject();
    // instead of:
    // var obj = new addon.Object();

Let's register our `createObject` method in `addon.cc`:

    // addon.cc
    #include <node.h>
    #include "myobject.h"

    using namespace v8;

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

    void InitAll(Handle<Object> exports, Handle<Object> module) {
      MyObject::Init();

      NODE_SET_METHOD(module, "exports", CreateObject);
    }

    NODE_MODULE(addon, InitAll)

In `myobject.h` we now introduce the static method `NewInstance` that takes care of instantiating the object (i.e. it does the job of `new` in JavaScript):

    // myobject.h
    #ifndef MYOBJECT_H
    #define MYOBJECT_H

    #include <node.h>
    #include <node_object_wrap.h>

    class MyObject : public node::ObjectWrap {
     public:
      static void Init();
      static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);

     private:
      explicit MyObject(double value = 0);
      ~MyObject();

      static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
      static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
      static v8::Persistent<v8::Function> constructor;
      double value_;
    };

    #endif

The implementation is similar to the above in `myobject.cc`:

    // myobject.cc
    #include <node.h>
    #include "myobject.h"

    using namespace v8;

    Persistent<Function> MyObject::constructor;

    MyObject::MyObject(double value) : value_(value) {
    }

    MyObject::~MyObject() {
    }

    void MyObject::Init() {
      Isolate* isolate = Isolate::GetCurrent();
      // Prepare constructor template
      Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
      tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
      tpl->InstanceTemplate()->SetInternalFieldCount(1);

      // Prototype
      NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

      constructor.Reset(isolate, tpl->GetFunction());
    }

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

      if (args.IsConstructCall()) {
        // Invoked as constructor: `new MyObject(...)`
        double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
        MyObject* obj = new MyObject(value);
        obj->Wrap(args.This());
        args.GetReturnValue().Set(args.This());
      } else {
        // Invoked as plain function `MyObject(...)`, turn into construct call.
        const int argc = 1;
        Local<Value> argv[argc] = { args[0] };
        Local<Function> cons = Local<Function>::New(isolate, constructor);
        args.GetReturnValue().Set(cons->NewInstance(argc, argv));
      }
    }

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

      const unsigned argc = 1;
      Handle<Value> argv[argc] = { args[0] };
      Local<Function> cons = Local<Function>::New(isolate, constructor);
      Local<Object> instance = cons->NewInstance(argc, argv);

      args.GetReturnValue().Set(instance);
    }

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

      MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
      obj->value_ += 1;

      args.GetReturnValue().Set(Number::New(isolate, obj->value_));
    }

Test it with:

    // test.js
    var createObject = require('./build/Release/addon');

    var obj = createObject(10);
    console.log( obj.plusOne() ); // 11
    console.log( obj.plusOne() ); // 12
    console.log( obj.plusOne() ); // 13

    var obj2 = createObject(20);
    console.log( obj2.plusOne() ); // 21
    console.log( obj2.plusOne() ); // 22
    console.log( obj2.plusOne() ); // 23

### Passing wrapped objects around#

In addition to wrapping and returning C++ objects, you can pass them around by unwrapping them with Node's `node::ObjectWrap::Unwrap` helper function. In the following `addon.cc` we introduce a function `add()` that can take on two `MyObject` objects:

    // addon.cc
    #include <node.h>
    #include <node_object_wrap.h>
    #include "myobject.h"

    using namespace v8;

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

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

      MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
          args[0]->ToObject());
      MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
          args[1]->ToObject());

      double sum = obj1->value() + obj2->value();
      args.GetReturnValue().Set(Number::New(isolate, sum));
    }

    void InitAll(Handle<Object> exports) {
      MyObject::Init();

      NODE_SET_METHOD(exports, "createObject", CreateObject);
      NODE_SET_METHOD(exports, "add", Add);
    }

    NODE_MODULE(addon, InitAll)

To make things interesting we introduce a public method in `myobject.h` so we can probe private values after unwrapping the object:

    // myobject.h
    #ifndef MYOBJECT_H
    #define MYOBJECT_H

    #include <node.h>
    #include <node_object_wrap.h>

    class MyObject : public node::ObjectWrap {
     public:
      static void Init();
      static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
      inline double value() const { return value_; }

     private:
      explicit MyObject(double value = 0);
      ~MyObject();

      static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
      static v8::Persistent<v8::Function> constructor;
      double value_;
    };

    #endif

The implementation of `myobject.cc` is similar as before:

    // myobject.cc
    #include <node.h>
    #include "myobject.h"

    using namespace v8;

    Persistent<Function> MyObject::constructor;

    MyObject::MyObject(double value) : value_(value) {
    }

    MyObject::~MyObject() {
    }

    void MyObject::Init() {
      Isolate* isolate = Isolate::GetCurrent();

      // Prepare constructor template
      Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
      tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
      tpl->InstanceTemplate()->SetInternalFieldCount(1);

      constructor.Reset(isolate, tpl->GetFunction());
    }

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

      if (args.IsConstructCall()) {
        // Invoked as constructor: `new MyObject(...)`
        double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
        MyObject* obj = new MyObject(value);
        obj->Wrap(args.This());
        args.GetReturnValue().Set(args.This());
      } else {
        // Invoked as plain function `MyObject(...)`, turn into construct call.
        const int argc = 1;
        Local<Value> argv[argc] = { args[0] };
        Local<Function> cons = Local<Function>::New(isolate, constructor);
        args.GetReturnValue().Set(cons->NewInstance(argc, argv));
      }
    }

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

      const unsigned argc = 1;
      Handle<Value> argv[argc] = { args[0] };
      Local<Function> cons = Local<Function>::New(isolate, constructor);
      Local<Object> instance = cons->NewInstance(argc, argv);

      args.GetReturnValue().Set(instance);
    }

Test it with:

    // test.js
    var addon = require('./build/Release/addon');

    var obj1 = addon.createObject(10);
    var obj2 = addon.createObject(20);
    var result = addon.add(obj1, obj2);

    console.log(result); // 30