I'm reading this article about V8 on HTML5Rocks. The article is old but I understand almost none of it and it bothers me. I'm taking this 1 step at a time but could someone help me with the Arrays section?

The article states:

Arrays

In order to handle large and sparse arrays, there are two types of array storage internally:

• Fast Elements: linear storage for pact key sets

• Dictionary Elements: hash table storage otherwise

It's best not to cause the array storage to flip from one type to another.

Question:

What would a Fast Elements linear storage array look like?

What would a Dictionary Elements hash table array look like?

For prevention purposes, how would I "flip from one type to another"?

I'm reading this article about V8 on HTML5Rocks. The article is old but I understand almost none of it and it bothers me. I'm taking this 1 step at a time but could someone help me with the Arrays section?

The article states:

Arrays

In order to handle large and sparse arrays, there are two types of array storage internally:

• Fast Elements: linear storage for pact key sets

• Dictionary Elements: hash table storage otherwise

It's best not to cause the array storage to flip from one type to another.

Question:

What would a Fast Elements linear storage array look like?

What would a Dictionary Elements hash table array look like?

For prevention purposes, how would I "flip from one type to another"?

• javascript
• arrays
• v8

• i think linear storage array would be normal array with just single elements and hash storage would be array of objects. – Akshay Commented Nov 22, 2016 at 5:46
• 1 The article does not speak of elements, but dictionary elements. You did not reflect the quote that well. – trincot Commented Nov 22, 2016 at 6:13
• @trincot sorry copy and mistake. – John Abraham Commented Nov 22, 2016 at 6:15
• 1 Ok, but now you have a trailing word Dictionary in the first option. – trincot Commented Nov 22, 2016 at 6:26

2 Answers 2

I will go a little other way round.

2) What would Dictionary Elements hash table Array look like?

A JavaScript object is a map from string to values. e.g.

var obj = {
  "name": "Sherlock Holmes",
  "address": "221B Baker Street"
}

V8 uses hash tables to represent objects unless using an optimized representation for special cases. This is much like a dictionary uses (words, meaning) pair.

Now, this hash table access is slow because initially all the keys and values in a hash table are undefined. On inserting a new pair, a hash is puted and the pair inserted at the insertion index. If there's already a key at that index, attempt to insert at next one and so on.

1) What would Fast Elements Linear storage Array look like?

In V8, an element is a property whose key is a non-negative integer (0, 1, 2, ...) i.e. a simple linear array whose properties can be accessed via a numerical index.

Fast elements are stored in a contiguous array. e.g.

var arr = [1, 2, 3];

They are a special case that is optimised for faster access as the index is already known and not to be puted.

3) For prevention purposes, How would I flip from one type to another?

For fast element, if you assign an index that's way past the end of the elements array, V8 may downgrade the elements to dictionary mode.

Reference: http://jayconrod./posts/52/a-tour-of-v8-object-representation

sir i can be wrong but according to your question what i have observed is explained

when we initialise an array it internally gets key as 0,1,2....etc as i pushed element with value its into array but array does not consider it

ex :

var arr = new Array()

arr[0] = 1
arr[1] = 2
arr[2] = "myname";
arr['myname'] = nick;

but when i do arr.length i get 3 so it does not consider the key apart from numeric but if i write arr[3] = {myname:'nick'} then it consider it as elements.

internally i think to keep the linear array different it looks for '{}'