How V8 handles this/dict?

From V8 presentations, I know that it optimizes constructions like this with tagging a type object:

function Point(x, y) {
  this.x = x;
  this.y = y;
}

What happens in the case I return an object (JSON), without using this? Will V8 apply the same optimization?

function parse() {
  ...
  // Suppose error happened (some branch of logic):
  return {
    pos: i,
    message: 'ka-boom'
  };
  ...
  // Everything is OK:
  return {
    value: 42
  };
}

In the sample above, will V8 tag with types objects (so, member access have chance to be real fast) or leave as maps? Note different paths return different objects.

Solution

Object literals get same optimization for the properties that are introduced inside the literal (not outside).

Constructed objects right now additionally get up to 8 properties that can be introduced outside without penalty. This means subclasses can call parent constructor without worry for up to 8 properties. However optional introduction of properties is still bad of course.

However, if your object literals require functions or other immutable heavy-weight data, not using prototypes will cost you dearly.

Also in these cases the object will not yet necessarily become a hash table but the properties will just be placed in an external array. This is slower than Java/C++-like objects which they are in fastest mode but not as slow as the slowest hash table mode.

So there is 3 modes of storage: fast in-object (like C++/Java objects), fastish out-of-object (no example), slow hashtable (like PHP, Python, Ruby in their canonical implementations)