doc: update ObjectWrap example

* Remove the global static reference to the constructor
* Use `Napi::Env::SetInstanceData` to store the constructor, and
* Add a static method that uses `Napi::FunctionReference::New` to
  create a new instance of the class by retrieving the constructor
  using `Napi::Env::GetInstanceData` and using
  `Napi::FunctionReference::New` to create the new instance.

Fixes: #711

doc/object_wrap.md

@@ -22,49 +22,57 @@ your C++ class methods.
 class Example : public Napi::ObjectWrap<Example> {
   public:
     static Napi::Object Init(Napi::Env env, Napi::Object exports);
-    Example(const Napi::CallbackInfo &info);
+    Example(const Napi::CallbackInfo& info);
+    static Napi::Value CreateNewItem(const Napi::CallbackInfo& info);
 
   private:
-    static Napi::FunctionReference constructor;
     double _value;
-    Napi::Value GetValue(const Napi::CallbackInfo &info);
-    Napi::Value SetValue(const Napi::CallbackInfo &info);
+    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")
+        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);
-    // Call the SuppressDestruct() method on the static data prevent the calling
-    // to this destructor to reset the reference when the environment is no longer
-    // available.
-    constructor.SuppressDestruct();
+    *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) {
+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::FunctionReference Example::constructor;
-
-Napi::Value Example::GetValue(const Napi::CallbackInfo &info){
+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::Value Example::SetValue(const Napi::CallbackInfo& info){
     Napi::Env env = info.Env();
     // ...
     Napi::Number value = info[0].As<Napi::Number>();
@@ -78,6 +86,16 @@ Napi::Object Init (Napi::Env env, Napi::Object 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)
 ```