## 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. - NAN_GC_CALLBACK() - Nan::AddGCEpilogueCallback() - Nan::RemoveGCEpilogueCallback() - Nan::AddGCPrologueCallback() - Nan::RemoveGCPrologueCallback() - Nan::GetHeapStatistics() - Nan::SetCounterFunction() - Nan::SetCreateHistogramFunction() - Nan::SetAddHistogramSampleFunction() - Nan::IdleNotification() - Nan::LowMemoryNotification() - Nan::ContextDisposedNotification() - Nan::GetInternalFieldPointer() - Nan::SetInternalFieldPointer() - Nan::AdjustExternalMemory() ### 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 callback; NAN_GC_CALLBACK(gcPrologueCallback) { v8::Local argv[] = { Nan::New("prologue").ToLocalChecked() }; Nan::MakeCallback(Nan::GetCurrentContext()->Global(), Nan::New(callback), 1, argv); } NAN_METHOD(Hook) { callback.Reset(To(args[0]).ToLocalChecked()); Nan::AddGCPrologueCallback(gcPrologueCallback); info.GetReturnValue().Set(info.Holder()); } ``` ### 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). ### 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). ### 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). ### 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). ### 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). ### 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). ### 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). ### 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). ### 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. ### 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). ### 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). ### Nan::GetInternalFieldPointer() Gets a pointer to the internal field with at `index` from a V8 `Object` handle. Signature: ```c++ void* Nan::GetInternalFieldPointer(v8::Local 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. ### Nan::SetInternalFieldPointer() Sets the value of the internal field at `index` on a V8 `Object` handle. Signature: ```c++ void Nan::SetInternalFieldPointer(v8::Local 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. ### 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).