callback, [`napi_unwrap`][] obtains the C++ instance that is the target of

the call.

For wrapped objects it may be difficult to distinguish between a function

called on a class prototype and a function called on an instance of a class.

A common pattern used to address this problem is to save a persistent

reference to the class constructor for later `instanceof` checks.

As an example:

```C

napi_value MyClass_constructor = nullptr;

status = napi_get_reference_value(env, MyClass::es_constructor, &MyClass_constructor);

assert(napi_ok == status);

bool is_instance = false;

status = napi_instanceof(env, es_this, MyClass_constructor, &is_instance);

assert(napi_ok == status);

if (is_instance) {

// napi_unwrap() ...

} else {

// otherwise...

}

```

The reference must be freed once it is no longer needed.

### *napi_define_class*

<!-- YAML

added: v8.0.0