some research notes for use in a blog post. trying to understand how 'undefined' is implemented in the v8 engine

gistfile1.cpp

//objects-inl.h
// undefined is a Primitive, which inherits from Object 
bool Object::IsUndefined() {
  return IsOddball() && Oddball::cast(this)->kind() == Oddball::kUndefined;
}

// what's an Oddball and where is kUndefined defined and what is it set to?

/*
objects.h:8213
Oddball class description 
Oddball : HeapObject
The Oddball describes objects null, undefined, true, and false.
*/

//objects.h:8247
static const byte kUndefined = 5; // generally unsure is 5 is somehow significant 

//include/v8.h:4301
static const int kUndefinedValueRootIndex = 5; // not sure why this is defined twice, and this time it is an int



//factory.cc:1476
Handle<Object> Factory::GlobalConstantFor(Handle<String> name) {
  Heap* h = isolate()->heap();
  if (name->Equals(h->undefined_string())) return undefined_value(); // look here
  if (name->Equals(h->nan_string())) return nan_value();
  if (name->Equals(h->infinity_string())) return infinity_value();
  return Handle<Object>::null();
}

// and where are undefined_string and undefined_value?
//heap.h:56
V(Oddball, undefined_value, UndefinedValue) 
//heap.h:221
V(undefined_string, "undefined")  // this must be where typeof x == "undefined" comes from