562 lines
21 KiB
Markdown
562 lines
21 KiB
Markdown
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# Object Wrap
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Class `Napi::ObjectWrap<T>` inherits from class [`Napi::InstanceWrap<T>`][].
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The `Napi::ObjectWrap<T>` class is used to bind the lifetime of C++ code to a
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JavaScript object. Once bound, each time an instance of the JavaScript object
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is created, an instance of the C++ class will also be created. When a method
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is called on the JavaScript object which is defined as an InstanceMethod, the
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corresponding C++ method on the wrapped C++ class will be invoked.
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In order to create a wrapper it's necessary to extend the
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`Napi::ObjectWrap<T>` class which contains all the plumbing to connect
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JavaScript code with a C++ object. Classes extending `Napi::ObjectWrap` can be
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instantiated from JavaScript using the **new** operator, and their methods can
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be directly invoked from JavaScript. The **wrap** word refers to a way of
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grouping methods and state of the class because it will be necessary write
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custom code to bridge each of your C++ class methods.
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## Example
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```cpp
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#include <napi.h>
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class Example : public Napi::ObjectWrap<Example> {
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public:
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static Napi::Object Init(Napi::Env env, Napi::Object exports);
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Example(const Napi::CallbackInfo& info);
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static Napi::Value CreateNewItem(const Napi::CallbackInfo& info);
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private:
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double _value;
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Napi::Value GetValue(const Napi::CallbackInfo& info);
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Napi::Value SetValue(const Napi::CallbackInfo& info);
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};
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Napi::Object Example::Init(Napi::Env env, Napi::Object exports) {
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// This method is used to hook the accessor and method callbacks
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Napi::Function func = DefineClass(env, "Example", {
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InstanceMethod<&Example::GetValue>("GetValue"),
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InstanceMethod<&Example::SetValue>("SetValue"),
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StaticMethod<&Example::CreateNewItem>("CreateNewItem"),
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});
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Napi::FunctionReference* constructor = new Napi::FunctionReference();
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// Create a peristent reference to the class constructor. This will allow
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// a function called on a class prototype and a function
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// called on instance of a class to be distinguished from each other.
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*constructor = Napi::Persistent(func);
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exports.Set("Example", func);
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// Store the constructor as the add-on instance data. This will allow this
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// add-on to support multiple instances of itself running on multiple worker
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// threads, as well as multiple instances of itself running in different
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// contexts on the same thread.
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//
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// By default, the value set on the environment here will be destroyed when
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// the add-on is unloaded using the `delete` operator, but it is also
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// possible to supply a custom deleter.
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env.SetInstanceData<Napi::FunctionReference>(constructor);
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return exports;
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}
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Example::Example(const Napi::CallbackInfo& info) :
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Napi::ObjectWrap<Example>(info) {
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Napi::Env env = info.Env();
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// ...
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Napi::Number value = info[0].As<Napi::Number>();
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this->_value = value.DoubleValue();
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}
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Napi::Value Example::GetValue(const Napi::CallbackInfo& info){
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Napi::Env env = info.Env();
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return Napi::Number::New(env, this->_value);
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}
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Napi::Value Example::SetValue(const Napi::CallbackInfo& info){
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Napi::Env env = info.Env();
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// ...
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Napi::Number value = info[0].As<Napi::Number>();
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this->_value = value.DoubleValue();
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return this->GetValue(info);
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}
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// Initialize native add-on
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Napi::Object Init (Napi::Env env, Napi::Object exports) {
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Example::Init(env, exports);
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return exports;
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}
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// Create a new item using the constructor stored during Init.
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Napi::Value Example::CreateNewItem(const Napi::CallbackInfo& info) {
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// Retrieve the instance data we stored during `Init()`. We only stored the
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// constructor there, so we retrieve it here to create a new instance of the
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// JS class the constructor represents.
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Napi::FunctionReference* constructor =
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info.Env().GetInstanceData<Napi::FunctionReference>();
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return constructor->New({ Napi::Number::New(info.Env(), 42) });
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}
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// Register and initialize native add-on
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NODE_API_MODULE(NODE_GYP_MODULE_NAME, Init)
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```
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The above code can be used from JavaScript as follows:
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```js
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'use strict'
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const { Example } = require('bindings')('addon')
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const example = new Example(11)
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console.log(example.GetValue())
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// It prints 11
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example.SetValue(19)
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console.log(example.GetValue());
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// It prints 19
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```
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At initialization time, the `Napi::ObjectWrap::DefineClass()` method must be
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used to hook up the accessor and method callbacks. It takes a list of property
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descriptors, which can be constructed via the various static methods on the base
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class.
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When JavaScript code invokes the constructor, the constructor callback will
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create a new C++ instance and "wrap" it into the newly created JavaScript
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object.
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When JavaScript code invokes a method or a property accessor on the class the
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corresponding C++ callback function will be executed.
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For a wrapped object it could be difficult to distinguish between a function
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called on a class prototype and a function called on instance of a class.
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Therefore it is good practice to save a persistent reference to the class
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constructor. This allows the two cases to be distinguished from each other by
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checking the this object against the class constructor.
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## Methods
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### Constructor
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Creates a new instance of a JavaScript object that wraps native instance.
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```cpp
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Napi::ObjectWrap(const Napi::CallbackInfo& callbackInfo);
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```
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- `[in] callbackInfo`: The object representing the components of the JavaScript
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request being made.
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### Unwrap
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Retrieves a native instance wrapped in a JavaScript object.
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```cpp
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static T* Napi::ObjectWrap::Unwrap(Napi::Object wrapper);
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```
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* `[in] wrapper`: The JavaScript object that wraps the native instance.
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Returns a native instance wrapped in a JavaScript object. Given the
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Napi:Object, this allows a method to get a pointer to the wrapped
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C++ object and then reference fields, call methods, etc. within that class.
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In many cases calling Unwrap is not required, as methods can
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use the `this` field for ObjectWrap when running in a method on a
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class that extends ObjectWrap.
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### DefineClass
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Defnines a JavaScript class with constructor, static and instance properties and
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methods.
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```cpp
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static Napi::Function Napi::ObjectWrap::DefineClass(Napi::Env env,
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const char* utf8name,
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const std::initializer_list<PropertyDescriptor>& properties,
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void* data = nullptr);
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```
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* `[in] env`: The environment in which to construct a JavaScript class.
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* `[in] utf8name`: Null-terminated string that represents the name of the
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JavaScript constructor function.
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* `[in] properties`: Initializer list of class property descriptor describing
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static and instance properties and methods of the class.
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See: [`Class property and descriptor`](class_property_descriptor.md).
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* `[in] data`: User-provided data passed to the constructor callback as `data`
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property of the `Napi::CallbackInfo`.
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Returns a `Napi::Function` representing the constructor function for the class.
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### DefineClass
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Defnines a JavaScript class with constructor, static and instance properties and
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methods.
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```cpp
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static Napi::Function Napi::ObjectWrap::DefineClass(Napi::Env env,
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const char* utf8name,
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const std::vector<PropertyDescriptor>& properties,
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void* data = nullptr);
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```
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* `[in] env`: The environment in which to construct a JavaScript class.
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* `[in] utf8name`: Null-terminated string that represents the name of the
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JavaScript constructor function.
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* `[in] properties`: Vector of class property descriptor describing static and
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instance properties and methods of the class.
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See: [`Class property and descriptor`](class_property_descriptor.md).
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* `[in] data`: User-provided data passed to the constructor callback as `data`
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property of the `Napi::CallbackInfo`.
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Returns a `Napi::Function` representing the constructor function for the class.
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### Finalize
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Provides an opportunity to run cleanup code that requires access to the
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`Napi::Env` before the wrapped native object instance is freed. Override to
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implement.
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```cpp
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virtual void Finalize(Napi::Env env);
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```
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- `[in] env`: `Napi::Env`.
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### StaticMethod
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Creates property descriptor that represents a static method of a JavaScript
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class.
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```cpp
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static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(
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const char* utf8name,
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StaticVoidMethodCallback method,
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napi_property_attributes attributes = napi_default,
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void* data = nullptr);
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```
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- `[in] utf8name`: Null-terminated string that represents the name of a static
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method for the class.
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- `[in] method`: The native function that represents a static method of a
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JavaScript class.
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- `[in] attributes`: The attributes associated with a particular property.
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One or more of `napi_property_attributes`.
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- `[in] data`: User-provided data passed into method when it is invoked.
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Returns `Napi::PropertyDescriptor` object that represents the static method of a
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JavaScript class.
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### StaticMethod
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Creates property descriptor that represents a static method of a JavaScript
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class.
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```cpp
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static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(
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const char* utf8name,
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StaticMethodCallback method,
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napi_property_attributes attributes = napi_default,
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void* data = nullptr);
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```
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- `[in] utf8name`: Null-terminated string that represents the name of a static
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method for the class.
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- `[in] method`: The native function that represents a static method of a
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JavaScript class.
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- `[in] attributes`: The attributes associated with a particular property.
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One or more of `napi_property_attributes`.
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- `[in] data`: User-provided data passed into method when it is invoked.
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Returns `Napi::PropertyDescriptor` object that represents a static method of a
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JavaScript class.
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### StaticMethod
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Creates property descriptor that represents a static method of a JavaScript
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class.
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```cpp
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static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(Symbol name,
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StaticVoidMethodCallback method,
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napi_property_attributes attributes = napi_default,
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void* data = nullptr);
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```
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- `[in] name`: Napi:Symbol that represents the name of a static
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method for the class.
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- `[in] method`: The native function that represents a static method of a
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JavaScript class.
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- `[in] attributes`: The attributes associated with a particular property.
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One or more of `napi_property_attributes`.
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- `[in] data`: User-provided data passed into method when it is invoked.
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Returns `Napi::PropertyDescriptor` object that represents the static method of a
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JavaScript class.
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### StaticMethod
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Creates property descriptor that represents a static method of a JavaScript
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class.
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```cpp
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static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(Symbol name,
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StaticMethodCallback method,
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napi_property_attributes attributes = napi_default,
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void* data = nullptr);
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```
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method for the class.
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- `[in] name`: Napi:Symbol that represents the name of a static.
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- `[in] method`: The native function that represents a static method of a
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JavaScript class.
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- `[in] attributes`: The attributes associated with a particular property.
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One or more of `napi_property_attributes`.
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- `[in] data`: User-provided data passed into method when it is invoked.
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Returns `Napi::PropertyDescriptor` object that represents a static method of a
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JavaScript class.
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### StaticMethod
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Creates property descriptor that represents a static method of a JavaScript
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class.
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```cpp
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template <StaticVoidMethodCallback method>
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static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(
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const char* utf8name,
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napi_property_attributes attributes = napi_default,
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void* data = nullptr);
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```
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- `[in] method`: The native function that represents a static method of a
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JavaScript class. This function returns nothing.
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- `[in] utf8name`: Null-terminated string that represents the name of a static
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method for the class.
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- `[in] attributes`: The attributes associated with a particular property.
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One or more of `napi_property_attributes`.
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- `[in] data`: User-provided data passed into method when it is invoked.
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Returns `Napi::PropertyDescriptor` object that represents the static method of a
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JavaScript class.
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### StaticMethod
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Creates property descriptor that represents a static method of a JavaScript
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class.
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```cpp
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template <StaticMethodCallback method>
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static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(
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const char* utf8name,
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napi_property_attributes attributes = napi_default,
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void* data = nullptr);
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```
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- `[in] method`: The native function that represents a static method of a
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JavaScript class.
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- `[in] utf8name`: Null-terminated string that represents the name of a static
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method for the class.
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- `[in] attributes`: The attributes associated with a particular property.
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One or more of `napi_property_attributes`.
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- `[in] data`: User-provided data passed into method when it is invoked.
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Returns `Napi::PropertyDescriptor` object that represents a static method of a
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JavaScript class.
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### StaticMethod
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Creates property descriptor that represents a static method of a JavaScript
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class.
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```cpp
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template <StaticVoidMethodCallback method>
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static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(Symbol name,
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napi_property_attributes attributes = napi_default,
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void* data = nullptr);
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```
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- `[in] method`: The native function that represents a static method of a
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JavaScript class.
|
||
|
|
- `[in] name`: Napi:Symbol that represents the name of a static
|
||
|
|
method for the class.
|
||
|
|
- `[in] attributes`: The attributes associated with a particular property.
|
||
|
|
One or more of `napi_property_attributes`.
|
||
|
|
- `[in] data`: User-provided data passed into method when it is invoked.
|
||
|
|
|
||
|
|
Returns `Napi::PropertyDescriptor` object that represents the static method of a
|
||
|
|
JavaScript class.
|
||
|
|
|
||
|
|
### StaticMethod
|
||
|
|
|
||
|
|
Creates property descriptor that represents a static method of a JavaScript
|
||
|
|
class.
|
||
|
|
|
||
|
|
```cpp
|
||
|
|
template <StaticMethodCallback method>
|
||
|
|
static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(Symbol name,
|
||
|
|
napi_property_attributes attributes = napi_default,
|
||
|
|
void* data = nullptr);
|
||
|
|
```
|
||
|
|
|
||
|
|
- `[in] method`: The native function that represents a static method of a
|
||
|
|
JavaScript class.
|
||
|
|
- `[in] name`: Napi:Symbol that represents the name of a static.
|
||
|
|
- `[in] attributes`: The attributes associated with a particular property.
|
||
|
|
One or more of `napi_property_attributes`.
|
||
|
|
- `[in] data`: User-provided data passed into method when it is invoked.
|
||
|
|
|
||
|
|
Returns `Napi::PropertyDescriptor` object that represents a static method of a
|
||
|
|
JavaScript class.
|
||
|
|
|
||
|
|
### StaticAccessor
|
||
|
|
|
||
|
|
Creates property descriptor that represents a static accessor property of a
|
||
|
|
JavaScript class.
|
||
|
|
|
||
|
|
```cpp
|
||
|
|
static Napi::PropertyDescriptor Napi::ObjectWrap::StaticAccessor(
|
||
|
|
const char* utf8name,
|
||
|
|
StaticGetterCallback getter,
|
||
|
|
StaticSetterCallback setter,
|
||
|
|
napi_property_attributes attributes = napi_default,
|
||
|
|
void* data = nullptr);
|
||
|
|
```
|
||
|
|
|
||
|
|
- `[in] utf8name`: Null-terminated string that represents the name of a static
|
||
|
|
accessor property for the class.
|
||
|
|
- `[in] getter`: The native function to call when a get access to the property
|
||
|
|
of a JavaScript class is performed.
|
||
|
|
- `[in] setter`: The native function to call when a set access to the property
|
||
|
|
of a JavaScript class is performed.
|
||
|
|
- `[in] attributes`: The attributes associated with a particular property.
|
||
|
|
One or more of `napi_property_attributes`.
|
||
|
|
- `[in] data`: User-provided data passed into getter or setter when
|
||
|
|
is invoked.
|
||
|
|
|
||
|
|
Returns `Napi::PropertyDescriptor` object that represents a static accessor
|
||
|
|
property of a JavaScript class.
|
||
|
|
|
||
|
|
### StaticAccessor
|
||
|
|
|
||
|
|
Creates property descriptor that represents a static accessor property of a
|
||
|
|
JavaScript class.
|
||
|
|
|
||
|
|
```cpp
|
||
|
|
static Napi::PropertyDescriptor Napi::ObjectWrap::StaticAccessor(Symbol name,
|
||
|
|
StaticGetterCallback getter,
|
||
|
|
StaticSetterCallback setter,
|
||
|
|
napi_property_attributes attributes = napi_default,
|
||
|
|
void* data = nullptr);
|
||
|
|
```
|
||
|
|
|
||
|
|
- `[in] name`: Napi:Symbol that represents the name of a static accessor.
|
||
|
|
- `[in] getter`: The native function to call when a get access to the property
|
||
|
|
of a JavaScript class is performed.
|
||
|
|
- `[in] setter`: The native function to call when a set access to the property
|
||
|
|
of a JavaScript class is performed.
|
||
|
|
- `[in] attributes`: The attributes associated with a particular property.
|
||
|
|
One or more of `napi_property_attributes`.
|
||
|
|
- `[in] data`: User-provided data passed into getter or setter when
|
||
|
|
is invoked.
|
||
|
|
|
||
|
|
Returns `Napi::PropertyDescriptor` object that represents a static accessor
|
||
|
|
property of a JavaScript class.
|
||
|
|
|
||
|
|
### StaticAccessor
|
||
|
|
|
||
|
|
Creates property descriptor that represents a static accessor property of a
|
||
|
|
JavaScript class.
|
||
|
|
|
||
|
|
```cpp
|
||
|
|
template <StaticGetterCallback getter, StaticSetterCallback setter=nullptr>
|
||
|
|
static Napi::PropertyDescriptor Napi::ObjectWrap::StaticAccessor(
|
||
|
|
const char* utf8name,
|
||
|
|
napi_property_attributes attributes = napi_default,
|
||
|
|
void* data = nullptr);
|
||
|
|
```
|
||
|
|
|
||
|
|
- `[in] getter`: The native function to call when a get access to the property
|
||
|
|
of a JavaScript class is performed.
|
||
|
|
- `[in] setter`: The native function to call when a set access to the property
|
||
|
|
of a JavaScript class is performed.
|
||
|
|
- `[in] utf8name`: Null-terminated string that represents the name of a static
|
||
|
|
accessor property for the class.
|
||
|
|
- `[in] attributes`: The attributes associated with a particular property.
|
||
|
|
One or more of `napi_property_attributes`.
|
||
|
|
- `[in] data`: User-provided data passed into getter or setter when
|
||
|
|
is invoked.
|
||
|
|
|
||
|
|
Returns `Napi::PropertyDescriptor` object that represents a static accessor
|
||
|
|
property of a JavaScript class.
|
||
|
|
|
||
|
|
### StaticAccessor
|
||
|
|
|
||
|
|
Creates property descriptor that represents a static accessor property of a
|
||
|
|
JavaScript class.
|
||
|
|
|
||
|
|
```cpp
|
||
|
|
template <StaticGetterCallback getter, StaticSetterCallback setter=nullptr>
|
||
|
|
static Napi::PropertyDescriptor Napi::ObjectWrap::StaticAccessor(Symbol name,
|
||
|
|
napi_property_attributes attributes = napi_default,
|
||
|
|
void* data = nullptr);
|
||
|
|
```
|
||
|
|
|
||
|
|
- `[in] getter`: The native function to call when a get access to the property
|
||
|
|
of a JavaScript class is performed.
|
||
|
|
- `[in] setter`: The native function to call when a set access to the property
|
||
|
|
of a JavaScript class is performed.
|
||
|
|
- `[in] name`: Napi:Symbol that represents the name of a static accessor.
|
||
|
|
- `[in] attributes`: The attributes associated with a particular property.
|
||
|
|
One or more of `napi_property_attributes`.
|
||
|
|
- `[in] data`: User-provided data passed into getter or setter when
|
||
|
|
is invoked.
|
||
|
|
|
||
|
|
Returns `Napi::PropertyDescriptor` object that represents a static accessor
|
||
|
|
property of a JavaScript class.
|
||
|
|
|
||
|
|
### StaticValue
|
||
|
|
|
||
|
|
Creates property descriptor that represents an static value property of a
|
||
|
|
JavaScript class.
|
||
|
|
```cpp
|
||
|
|
static Napi::PropertyDescriptor Napi::ObjectWrap::StaticValue(
|
||
|
|
const char* utf8name,
|
||
|
|
Napi::Value value,
|
||
|
|
napi_property_attributes attributes = napi_default);
|
||
|
|
```
|
||
|
|
|
||
|
|
- `[in] utf8name`: Null-terminated string that represents the name of the static
|
||
|
|
property.
|
||
|
|
- `[in] value`: The value that's retrieved by a get access of the property.
|
||
|
|
- `[in] attributes`: The attributes to be associated with the property in
|
||
|
|
addition to the napi_static attribute. One or more of
|
||
|
|
`napi_property_attributes`.
|
||
|
|
|
||
|
|
Returns `Napi::PropertyDescriptor` object that represents an static value
|
||
|
|
property of a JavaScript class
|
||
|
|
|
||
|
|
### StaticValue
|
||
|
|
|
||
|
|
Creates property descriptor that represents an static value property of a
|
||
|
|
JavaScript class.
|
||
|
|
```cpp
|
||
|
|
static Napi::PropertyDescriptor Napi::ObjectWrap::StaticValue(Symbol name,
|
||
|
|
Napi::Value value,
|
||
|
|
napi_property_attributes attributes = napi_default);
|
||
|
|
```
|
||
|
|
|
||
|
|
- `[in] name`: The `Napi::Symbol` object whose value is used to identify the
|
||
|
|
name of the static property.
|
||
|
|
- `[in] value`: The value that's retrieved by a get access of the property.
|
||
|
|
- `[in] attributes`: The attributes to be associated with the property in
|
||
|
|
addition to the napi_static attribute. One or more of
|
||
|
|
`napi_property_attributes`.
|
||
|
|
|
||
|
|
Returns `Napi::PropertyDescriptor` object that represents an static value
|
||
|
|
property of a JavaScript class
|
||
|
|
|
||
|
|
[`Napi::InstanceWrap<T>`]: ./instance_wrap.md
|