Object Wrap

The Napi::ObjectWrap class is used to bind the lifetime of C++ code to a JavaScript object. Once bound, each time an instance of the JavaScript object is created, an instance of the C++ class will also be created. When a method is called on the JavaScript object which is defined as an InstanceMethod, the corresponding C++ method on the wrapped C++ class will be invoked.

In order to create a wrapper it's necessary to extend the Napi::ObjectWrapclass which contains all the plumbing to connect JavaScript code with a C++ object. Classes extending Napi::ObjectWrap can be instantiated from JavaScript using the new operator, and their methods can be directly invoked from JavaScript. The wrap word refers to a way of grouping methods and state of the class because it will be necessary write custom code to bridge each of your C++ class methods.

Example

#include <napi.h>

class Example : public Napi::ObjectWrap<Example> {
  public:
    static Napi::Object Init(Napi::Env env, Napi::Object exports);
    Example(const Napi::CallbackInfo& info);
    static Napi::Value CreateNewItem(const Napi::CallbackInfo& info);

  private:
    double _value;
    Napi::Value GetValue(const Napi::CallbackInfo& info);
    Napi::Value SetValue(const Napi::CallbackInfo& info);
};

Napi::Object Example::Init(Napi::Env env, Napi::Object exports) {
    // This method is used to hook the accessor and method callbacks
    Napi::Function func = DefineClass(env, "Example", {
        InstanceMethod<&Example::GetValue>("GetValue"),
        InstanceMethod<&Example::SetValue>("SetValue"),
        StaticMethod<&Example::CreateNewItem>("CreateNewItem"),
    });

    Napi::FunctionReference* constructor = new Napi::FunctionReference();

    // Create a peristent reference to the class constructor. This will allow
    // a function called on a class prototype and a function
    // called on instance of a class to be distinguished from each other.
    *constructor = Napi::Persistent(func);
    exports.Set("Example", func);

    // Store the constructor as the add-on instance data. This will allow this
    // add-on to support multiple instances of itself running on multiple worker
    // threads, as well as multiple instances of itself running in different
    // contexts on the same thread.
    //
    // By default, the value set on the environment here will be destroyed when
    // the add-on is unloaded using the `delete` operator, but it is also
    // possible to supply a custom deleter.
    env.SetInstanceData<Napi::FunctionReference>(constructor);

    return exports;
}

Example::Example(const Napi::CallbackInfo& info) : Napi::ObjectWrap<Example>(info) {
    Napi::Env env = info.Env();
    // ...
    Napi::Number value = info[0].As<Napi::Number>();
    this->_value = value.DoubleValue();
}

Napi::Value Example::GetValue(const Napi::CallbackInfo& info){
    Napi::Env env = info.Env();
    return Napi::Number::New(env, this->_value);
}

Napi::Value Example::SetValue(const Napi::CallbackInfo& info){
    Napi::Env env = info.Env();
    // ...
    Napi::Number value = info[0].As<Napi::Number>();
    this->_value = value.DoubleValue();
    return this->GetValue(info);
}

// Initialize native add-on
Napi::Object Init (Napi::Env env, Napi::Object exports) {
    Example::Init(env, exports);
    return exports;
}

// Create a new item using the constructor stored during Init.
Napi::Value Example::CreateNewItem(const Napi::CallbackInfo& info) {
  // Retrieve the instance data we stored during `Init()`. We only stored the
  // constructor there, so we retrieve it here to create a new instance of the
  // JS class the constructor represents.
  Napi::FunctionReference* constructor =
      info.Env().GetInstanceData<Napi::FunctionReference>();
  return constructor->New({ Napi::Number::New(info.Env(), 42) });
}

// Register and initialize native add-on
NODE_API_MODULE(NODE_GYP_MODULE_NAME, Init)

The above code can be used from JavaScript as follows:

'use strict'

const { Example } = require('bindings')('addon')

const example = new Example(11)
console.log(example.GetValue())
// It prints 11
example.SetValue(19)
console.log(example.GetValue());
// It prints 19

At initialization time, the Napi::ObjectWrap::DefineClass() method must be used to hook up the accessor and method callbacks. It takes a list of property descriptors, which can be constructed via the various static methods on the base class.

When JavaScript code invokes the constructor, the constructor callback will create a new C++ instance and "wrap" it into the newly created JavaScript object.

When JavaScript code invokes a method or a property accessor on the class the corresponding C++ callback function will be executed.

For a wrapped object it could be difficult to distinguish between a function called on a class prototype and a function called on instance of a class. Therefore it is good practice to save a persistent reference to the class constructor. This allows the two cases to be distinguished from each other by checking the this object against the class constructor.

Methods

Constructor

Creates a new instance of a JavaScript object that wraps native instance.

Napi::ObjectWrap(const Napi::CallbackInfo& callbackInfo);

[in] callbackInfo: The object representing the components of the JavaScript request being made.

Unwrap

Retrieves a native instance wrapped in a JavaScript object.

static T* Napi::ObjectWrap::Unwrap(Napi::Object wrapper);

[in] wrapper: The JavaScript object that wraps the native instance.

Returns a native instance wrapped in a JavaScript object. Given the Napi:Object, this allows a method to get a pointer to the wrapped C++ object and then reference fields, call methods, etc. within that class. In many cases calling Unwrap is not required, as methods can use the this field for ObjectWrap when running in a method on a class that extends ObjectWrap.

DefineClass

Defnines a JavaScript class with constructor, static and instance properties and methods.

static Napi::Function Napi::ObjectWrap::DefineClass(Napi::Env env,
                                const char* utf8name,
                                const std::initializer_list<PropertyDescriptor>& properties,
                                void* data = nullptr);

[in] env: The environment in which to construct a JavaScript class.
[in] utf8name: Null-terminated string that represents the name of the JavaScript constructor function.
[in] properties: Initializer list of class property descriptor describing static and instance properties and methods of the class. See: Class property and descriptor.
[in] data: User-provided data passed to the constructor callback as data property of the Napi::CallbackInfo.

Returns a Napi::Function representing the constructor function for the class.

DefineClass

Defnines a JavaScript class with constructor, static and instance properties and methods.

static Napi::Function Napi::ObjectWrap::DefineClass(Napi::Env env,
                            const char* utf8name,
                            const std::vector<PropertyDescriptor>& properties,
                            void* data = nullptr);

[in] env: The environment in which to construct a JavaScript class.
[in] utf8name: Null-terminated string that represents the name of the JavaScript constructor function.
[in] properties: Vector of class property descriptor describing static and instance properties and methods of the class. See: Class property and descriptor.
[in] data: User-provided data passed to the constructor callback as data property of the Napi::CallbackInfo.

Returns a Napi::Function representing the constructor function for the class.

Finalize

Provides an opportunity to run cleanup code that requires access to the Napi::Env before the wrapped native object instance is freed. Override to implement.

virtual void Finalize(Napi::Env env);

[in] env: Napi::Env.