Add-on Structure

Creating add-ons that work correctly when loaded multiple times from the same source package into multiple Node.js threads and/or multiple times into the same Node.js thread requires that all global data they hold be associated with the environment in which they run. It is not safe to store global data in static variables because doing so does not take into account the fact that an add-on may be loaded into multiple threads nor that an add-on may be loaded multiple times into a single thread.

The Napi::Addon class can be used to define an entire add-on. Instances of Napi::Addon subclasses become instances of the add-on, stored safely by Node.js on its various threads and into its various contexts. Thus, any data stored in the instance variables of a Napi::Addon subclass instance are stored safely by Node.js. Functions exposed to JavaScript using Napi::Addon::InstanceMethod and/or Napi::Addon::DefineAddon are instance methods of the Napi::Addon subclass and thus have access to data stored inside the instance.

Napi::Addon::DefineProperties may be used to attach Napi::Addon subclass instance methods to objects other than the one that will be returned to Node.js as the add-on instance.

The Napi::Addon class can be used together with the NODE_API_ADDON() and NODE_API_NAMED_ADDON() macros to define add-ons.

Example

#include <napi.h>

class ExampleAddon : public Napi::Addon<ExampleAddon> {
 public:
  ExampleAddon(Napi::Env env, Napi::Object exports) {
    // In the constructor we declare the functions the add-on makes avaialable
    // to JavaScript.
    DefineAddon(exports, {
      InstanceMethod("increment", &ExampleAddon::Increment),

      // We can also attach plain objects to `exports`, and instance methods as
      // properties of those sub-objects.
      InstanceValue("subObject", DefineProperties(Napi::Object::New(), {
        InstanceMethod("decrement", &ExampleAddon::Decrement
      })), napi_enumerable)
    });
  }
 private:

  // This method has access to the data stored in the environment because it is
  // an instance method of `ExampleAddon` and because it was listed among the
  // property descriptors passed to `DefineAddon()` in the constructor.
  Napi::Value Increment(const Napi::CallbackInfo& info) {
    return Napi::Number::New(info.Env(), ++value);
  }

  // This method has access to the data stored in the environment because it is
  // an instance method of `ExampleAddon` and because it was exposed to
  // JavaScript by calling `DefineProperties()` with the object onto which it is
  // attached.
  Napi::Value Decrement(const Napi::CallbackInfo& info) {
    return Napi::Number::New(info.Env(), --value);
  }

  // Data stored in these variables is unique to each instance of the add-on.
  uint32_t value = 42;
};

// The macro announces that instances of the class `ExampleAddon` will be
// created for each instance of the add-on that must be loaded into Node.js.
NODE_API_ADDON(ExampleAddon)

The above code can be used from JavaScript as follows:

'use strict'

const exampleAddon = require('bindings')('example_addon');
console.log(exampleAddon.increment()); // prints 43
console.log(exampleAddon.increment()); // prints 44
consnole.log(exampleAddon.subObject.decrement()); // prints 43

When Node.js loads an instance of the add-on, a new instance of the class is created. Its constructor receives the environment Napi::Env env and the exports object Napi::Object exports. It can then use the method DefineAddon to either attach methods, accessors, and/or values to the exports object or to create its own exports object and attach methods, accessors, and/or values to it.

Functions created with Napi::Function::New(), accessors created with PropertyDescriptor::Accessor(), and values can also be attached. If their implementation requires the ExampleAddon instance, it can be retrieved from the Napi::Env env with GetInstanceData():

void ExampleBinding(const Napi::CallbackInfo& info) {
  ExampleAddon* addon = info.Env().GetInstanceData<ExampleAddon>();
}

Methods

Constructor

Creates a new instance of the add-on.

Napi::Addon(Napi::Env env, Napi::Object exports);

[in] env: The environment into which the add-on is being loaded.
[in] exports: The exports object received from JavaScript.

Typically, the constructor calls DefineAddon() to attach methods, accessors, and/or values to exports. The constructor may also create a new object and pass it to DefineAddon() as its first parameter if it wishes to replace the exports object as provided by Node.js.

DefineAddon

Defines an add-on instance with functions, accessors, and/or values.

void Napi::Addon::DefineAddon(Napi::Object exports,
                   const std::initializer_list<PropertyDescriptor>& properties);

[in] exports: The object to return to Node.js as an instance of the add-on.
[in] properties: Initializer list of add-on property descriptors of the methods, property accessors, and values that define the add-on. They will be set on exports. See: Class property and descriptor.

DefineProperties

Defines function, accessor, and/or value properties on an object using add-on instance methods.

Napi::Object
Napi::Addon::DefineProperties(Napi::Object object,
                   const std::initializer_list<PropertyDescriptor>& properties);

[in] object: The object that will receive the new properties.
[in] properties: Initializer list of property descriptors of the methods, property accessors, and values to attach to object. See: Class property and descriptor.

Returns object.