const o = ['a', 'b', 'c']
console.log(o[1])          // 'b'
delete o[1]
console.log(o[1])          // undefined
o.__proto__ = { 1: 'B' }
console.log(o[0])          // 'a'
console.log(o[1])          // 'B'   但如何确定要访问原型链？？🤔
console.log(o[2])          // 'c'
console.log(o[3])          // undefined

const arr = [1, 2, 3]
arr[1999] = 1999
// arr 会如何存储？

const testArr = [1, "hello", true, function () {
  return 1;
}];

// path:v8/src/objects/js-objects-inl.h
// If the fast-case backing storage takes up much more memory than a dictionary
// backing storage would, the object should have slow elements.
// static
static inline bool ShouldConvertToSlowElements(uint32_t used_elements,
                                               uint32_t new_capacity) {
  uint32_t size_threshold = NumberDictionary::kPreferFastElementsSizeFactor *
                            NumberDictionary::ComputeCapacity(used_elements) *
                            NumberDictionary::kEntrySize;
  return size_threshold <= new_capacity;
}
static inline bool ShouldConvertToSlowElements(JSObject object,
                                               uint32_t capacity,
                                               uint32_t index,
                                               uint32_t* new_capacity) {
  STATIC_ASSERT(JSObject::kMaxUncheckedOldFastElementsLength <=
                JSObject::kMaxUncheckedFastElementsLength);
  if (index < capacity) {
    *new_capacity = capacity;
    return false;
  }
  if (index - capacity >= JSObject::kMaxGap) return true;
  *new_capacity = JSObject::NewElementsCapacity(index + 1);
  DCHECK_LT(index, *new_capacity);
  if (*new_capacity <= JSObject::kMaxUncheckedOldFastElementsLength ||
      (*new_capacity <= JSObject::kMaxUncheckedFastElementsLength &&
       ObjectInYoungGeneration(object))) {
    return false;
  }
  return ShouldConvertToSlowElements(object.GetFastElementsUsage(),
                                     *new_capacity);
}

const arr = [1, 2, 3];
arr[1999] = 1999;
%DebugPrint(arr);

// path:v8/src/objects/js-objects.cc
// line:4932
static bool ShouldConvertToFastElements(JSObject object,
                                        NumberDictionary dictionary,
                                        uint32_t index,
                                        uint32_t* new_capacity) {
  // If properties with non-standard attributes or accessors were added, we
  // cannot go back to fast elements.
  if (dictionary.requires_slow_elements()) return false;
  // Adding a property with this index will require slow elements.
  if (index >= static_cast<uint32_t>(Smi::kMaxValue)) return false;
  if (object.IsJSArray()) {
    Object length = JSArray::cast(object).length();
    if (!length.IsSmi()) return false;
    *new_capacity = static_cast<uint32_t>(Smi::ToInt(length));
  } else if (object.IsJSArgumentsObject()) {
    return false;
  } else {
    *new_capacity = dictionary.max_number_key() + 1;
  }
  *new_capacity = std::max(index + 1, *new_capacity);
  uint32_t dictionary_size = static_cast<uint32_t>(dictionary.Capacity()) *
                             NumberDictionary::kEntrySize;
  // 看这里👇， 当慢数组转换成快数组能节省 不少于 50% 的空间时，才会将其转换
  // Turn fast if the dictionary only saves 50% space.
  return 2 * dictionary_size >= *new_capacity;
}

// path:v8/src/objects/js-array.h
// Dispatched behavior.
DECL_PRINTER(JSArray)
DECL_VERIFIER(JSArray)
// Number of element slots to pre-allocate for an empty array.
// 空数组预分配的大小为4
static const int kPreallocatedArrayElements = 4;
static const int kLengthDescriptorIndex = 0;

// path:v8/src/objects/js-objects.h
// line:551👇
static const uint32_t kMinAddedElementsCapacity = 16;
// Computes the new capacity when expanding the elements of a JSObject.
static uint32_t NewElementsCapacity(uint32_t old_capacity) {
  // (old_capacity + 50%) + kMinAddedElementsCapacity
  // 扩容公式:原有内存容量（1.5倍）+ 16
  return old_capacity + (old_capacity >> 1) + kMinAddedElementsCapacity;
}

const arr = [1, 2, 3, 4];
arr.push(5);
%DebugPrint(arr);

// path:src/objects/elements.cc
// line:750
if (2 * length + JSObject::kMinAddedElementsCapacity <= capacity) {
  // If more than half the elements won't be used, trim the array.
  // Do not trim from short arrays to prevent frequent trimming on
  // repeated pop operations.
  // Leave some space to allow for subsequent push operations.
  int elements_to_trim = length + 1 == old_length
                             ? (capacity - length) / 2
                             : capacity - length;
  isolate->heap()->RightTrimFixedArray(*backing_store, elements_to_trim);
  // Fill the non-trimmed elements with holes.
  BackingStore::cast(*backing_store)
      .FillWithHoles(length,
                     std::min(old_length, capacity - elements_to_trim));
} else {
  // Otherwise, fill the unused tail with holes.
  BackingStore::cast(*backing_store).FillWithHoles(length, old_length);
}