How are objects represented internally in V8?

Question

I always thought that object in javascript are simple key-value pair i.e. hash tables, but after reading this article I am confused.

The author specially says:

when you use an object as if it was a hash table, it will be turned into a hash table.

So, what the hell was it before? specially the whole para got me confused. Can somebody shed some light on?

Answer 1

According to this answer, in V8 there are two modes an object can have—Dictionary mode and Fast mode.

Objects are originally in fast mode where no hash map—no computation—is required for property access. It stores objects like structs in C. It's only once you start using an "object as if it was a hash table, it will be turned into a hash table"—dictionary mode. Once this happens, you get the performance penalty of a hash map being used behind the scenes for property access.

For example:

// running node with `--allow-natives-syntax` flag

var obj = { a: true, b: false };
%HasFastProperties(obj); // true (Fast mode)
delete obj.a;
%HasFastProperties(obj); // false (Dictionary mode)

Or:

var obj = { a: true, b: false };
%HasFastProperties(obj); // true (Fast mode)
// add lots of properties
for (var i = 0; i < 100; i++) {
    obj["prop" + i] = i;
}
%HasFastProperties(obj); // false (Dictionary mode)

The reason it goes into dictionary mode when doing this is a performance optimization. It's faster to add/remove properties in dictionary mode than fast mode and so the V8 engine optimizes for changing structure instead of property access when it detects this behaviour (Read more here).