class_property_descriptor.md

Class property and descriptor

Property descriptor for use with Napi::ObjectWrap<T> and Napi::InstanceWrap<T>. This is different from the standalone Napi::PropertyDescriptor because it is specific to each Napi::ObjectWrap<T> and Napi::InstanceWrap<T> subclasses. This prevents using descriptors from a different class when defining a new class (preventing the callbacks from having incorrect this pointers).

Napi::ClassPropertyDescriptor is a helper class created with Napi::ObjectWrap<T> and Napi::InstanceWrap<T>. For more reference about it see:

• InstanceWrap
• ObjectWrap

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);

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

Napi::Object Example::Init(Napi::Env env, Napi::Object exports) {
    Napi::Function func = DefineClass(env, "Example", {
        // Register a class instance accessor with getter and setter functions.
        InstanceAccessor<&Example::GetValue, &Example::SetValue>("value"),
        // We can also register a readonly accessor by omitting the setter.
        InstanceAccessor<&Example::GetValue>("readOnlyProp")
    });

    Napi::FunctionReference *constructor = new Napi::FunctionReference();
    *constructor = Napi::Persistent(func);
    env.SetInstanceData(constructor);
    exports.Set("Example", func);

    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);
}

void Example::SetValue(const Napi::CallbackInfo &info, const Napi::Value &value) {
    Napi::Env env = info.Env();
    // ...
    Napi::Number arg = value.As<Napi::Number>();
    this->_value = arg.DoubleValue();
}

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

// 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.value);
// It prints 11
example.value = 19;
console.log(example.value);
// It prints 19
example.readOnlyProp = 500;
console.log(example.readOnlyProp);
// Unchanged. It prints 19

Methods

Constructor

Creates new instance of Napi::ClassPropertyDescriptor descriptor object.

Napi::ClassPropertyDescriptor(napi_property_descriptor desc) : _desc(desc) {}

• [in] desc: The napi_property_descriptor

Returns new instance of Napi::ClassPropertyDescriptor that is used as property descriptor inside the Napi::ObjectWrap<T> class.

Operator

operator napi_property_descriptor&() { return _desc; }

Returns the original Node-API napi_property_descriptor wrapped inside the Napi::ClassPropertyDescriptor

operator const napi_property_descriptor&() const { return _desc; }

Returns the original Node-API napi_property_descriptor wrapped inside the Napi::ClassPropertyDescriptor