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doc: document common pattern for instanceof checks #16699

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doc: document common pattern for instanceof checks
mhdawson Nov 2, 2017
dc5fee6
squash: minor fixup
mhdawson Nov 2, 2017
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squash: address comments
mhdawson Nov 17, 2017
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doc: document common pattern for instanceof checks 
Fixes: #13824
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@mhdawson
mhdawson committed Nov 2, 2017
commit 58e47e8a3c8c1c1020bb210e3b0d78a9c35cf4ea
23 changes: 23 additions & 0 deletions doc/api/n-api.md
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Expand Up @@ -3021,6 +3021,29 @@ constructor and methods can be called from JavaScript.
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:

```
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() ...
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@cjihrig cjihrig Nov 3, 2017

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Should indentation be two spaces here and a couple lines down?

} else {
// otherwise...
}
```

Of course you also need to make sure to free the reference once it is no longer needed.

### *napi_define_class*
<!-- YAML
added: v8.0.0
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