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v8_internals.md
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## V8 internals
The hooks to access V8 internals—including GC and statistics—are different across the supported versions of V8, therefore NAN provides its own hooks that call the appropriate V8 methods.
- <a href="#api_nan_gc_callback"><b><code>NAN_GC_CALLBACK()</code></b></a>
- <a href="#api_nan_add_gc_epilogue_callback"><b><code>Nan::AddGCEpilogueCallback()</code></b></a>
- <a href="#api_nan_remove_gc_epilogue_callback"><b><code>Nan::RemoveGCEpilogueCallback()</code></b></a>
- <a href="#api_nan_add_gc_prologue_callback"><b><code>Nan::AddGCPrologueCallback()</code></b></a>
- <a href="#api_nan_remove_gc_prologue_callback"><b><code>Nan::RemoveGCPrologueCallback()</code></b></a>
- <a href="#api_nan_get_heap_statistics"><b><code>Nan::GetHeapStatistics()</code></b></a>
- <a href="#api_nan_set_counter_function"><b><code>Nan::SetCounterFunction()</code></b></a>
- <a href="#api_nan_set_create_histogram_function"><b><code>Nan::SetCreateHistogramFunction()</code></b></a>
- <a href="#api_nan_set_add_histogram_sample_function"><b><code>Nan::SetAddHistogramSampleFunction()</code></b></a>
- <a href="#api_nan_idle_notification"><b><code>Nan::IdleNotification()</code></b></a>
- <a href="#api_nan_low_memory_notification"><b><code>Nan::LowMemoryNotification()</code></b></a>
- <a href="#api_nan_context_disposed_notification"><b><code>Nan::ContextDisposedNotification()</code></b></a>
- <a href="#api_nan_get_internal_field_pointer"><b><code>Nan::GetInternalFieldPointer()</code></b></a>
- <a href="#api_nan_set_internal_field_pointer"><b><code>Nan::SetInternalFieldPointer()</code></b></a>
- <a href="#api_nan_adjust_external_memory"><b><code>Nan::AdjustExternalMemory()</code></b></a>
<a name="api_nan_gc_callback"></a>
### NAN_GC_CALLBACK(callbackname)
Use `NAN_GC_CALLBACK` to declare your callbacks for `Nan::AddGCPrologueCallback()` and `Nan::AddGCEpilogueCallback()`. Your new method receives the arguments `v8::GCType type` and `v8::GCCallbackFlags flags`.
```c++
static Nan::Persistent<Function> callback;
NAN_GC_CALLBACK(gcPrologueCallback) {
v8::Local<Value> argv[] = { Nan::New("prologue").ToLocalChecked() };
Nan::MakeCallback(Nan::GetCurrentContext()->Global(), Nan::New(callback), 1, argv);
}
NAN_METHOD(Hook) {
callback.Reset(To<Function>(args[0]).ToLocalChecked());
Nan::AddGCPrologueCallback(gcPrologueCallback);
info.GetReturnValue().Set(info.Holder());
}
```
<a name="api_nan_add_gc_epilogue_callback"></a>
### Nan::AddGCEpilogueCallback()
Signature:
```c++
void Nan::AddGCEpilogueCallback(v8::Isolate::GCEpilogueCallback callback, v8::GCType gc_type_filter = v8::kGCTypeAll)
```
Calls V8's [`AddGCEpilogueCallback()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#a90d1860babc76059c62514b422f56960).
<a name="api_nan_remove_gc_epilogue_callback"></a>
### Nan::RemoveGCEpilogueCallback()
Signature:
```c++
void Nan::RemoveGCEpilogueCallback(v8::Isolate::GCEpilogueCallback callback)
```
Calls V8's [`RemoveGCEpilogueCallback()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#a05c60859fd4b8e96bfcd451281ed6c7c).
<a name="api_nan_add_gc_prologue_callback"></a>
### Nan::AddGCPrologueCallback()
Signature:
```c++
void Nan::AddGCPrologueCallback(v8::Isolate::GCPrologueCallback, v8::GCType gc_type_filter callback)
```
Calls V8's [`AddGCPrologueCallback()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#ab4b87b8f9f8e5bf95eba4009357e001f).
<a name="api_nan_remove_gc_prologue_callback"></a>
### Nan::RemoveGCPrologueCallback()
Signature:
```c++
void Nan::RemoveGCPrologueCallback(v8::Isolate::GCPrologueCallback callback)
```
Calls V8's [`RemoveGCEpilogueCallback()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#a9f6c51932811593f81ff30b949124186).
<a name="api_nan_get_heap_statistics"></a>
### Nan::GetHeapStatistics()
Signature:
```c++
void Nan::GetHeapStatistics(v8::HeapStatistics *heap_statistics)
```
Calls V8's [`GetHeapStatistics()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#a5593ac74687b713095c38987e5950b34).
<a name="api_nan_set_counter_function"></a>
### Nan::SetCounterFunction()
Signature:
```c++
void Nan::SetCounterFunction(v8::CounterLookupCallback cb)
```
Calls V8's [`SetCounterFunction()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#a045d7754e62fa0ec72ae6c259b29af94).
<a name="api_nan_set_create_histogram_function"></a>
### Nan::SetCreateHistogramFunction()
Signature:
```c++
void Nan::SetCreateHistogramFunction(v8::CreateHistogramCallback cb)
```
Calls V8's [`SetCreateHistogramFunction()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#a542d67e85089cb3f92aadf032f99e732).
<a name="api_nan_set_add_histogram_sample_function"></a>
### Nan::SetAddHistogramSampleFunction()
Signature:
```c++
void Nan::SetAddHistogramSampleFunction(v8::AddHistogramSampleCallback cb)
```
Calls V8's [`SetAddHistogramSampleFunction()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#aeb420b690bc2c216882d6fdd00ddd3ea).
<a name="api_nan_idle_notification"></a>
### Nan::IdleNotification()
Signature:
```c++
void Nan::IdleNotification(v8::HeapStatistics *heap_statistics)
```
Calls V8's [`IdleNotification()` or `IdleNotificationDeadline()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#ad6a2a02657f5425ad460060652a5a118) depending on V8 version.
<a name="api_nan_low_memory_notification"></a>
### Nan::LowMemoryNotification()
Signature:
```c++
void Nan::LowMemoryNotification()
```
Calls V8's [`LowMemoryNotification()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#a24647f61d6b41f69668094bdcd6ea91f).
<a name="api_nan_context_disposed_notification"></a>
### Nan::ContextDisposedNotification()
Signature:
```c++
void Nan::ContextDisposedNotification()
```
Calls V8's [`ContextDisposedNotification()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#ad7f5dc559866343fe6cd8db1f134d48b).
<a name="api_nan_get_internal_field_pointer"></a>
### Nan::GetInternalFieldPointer()
Gets a pointer to the internal field with at `index` from a V8 `Object` handle.
Signature:
```c++
void* Nan::GetInternalFieldPointer(v8::Local<v8::Object> object, int index)
```
Calls the Object's [`GetAlignedPointerFromInternalField()` or `GetPointerFromInternalField()`](https://v8docs.nodesource.com/io.js-3.3/db/d85/classv8_1_1_object.html#ab3c57184263cf29963ef0017bec82281) depending on the version of V8.
<a name="api_nan_set_internal_field_pointer"></a>
### Nan::SetInternalFieldPointer()
Sets the value of the internal field at `index` on a V8 `Object` handle.
Signature:
```c++
void Nan::SetInternalFieldPointer(v8::Local<v8::Object> object, int index, void* value)
```
Calls the Object's [`SetAlignedPointerInInternalField()` or `SetPointerInInternalField()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#ad7f5dc559866343fe6cd8db1f134d48b) depending on the version of V8.
<a name="api_nan_adjust_external_memory"></a>
### Nan::AdjustExternalMemory()
Signature:
```c++
int Nan::AdjustExternalMemory(int bytesChange)
```
Calls V8's [`AdjustAmountOfExternalAllocatedMemory()`](https://v8docs.nodesource.com/io.js-3.3/d5/dda/classv8_1_1_isolate.html#ae1a59cac60409d3922582c4af675473e).

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