v8性能准则

Question

I tend to use the factory method to construct my objects in javascript without the this keyword. For example:

var PointA = function(x, y) {
  var z = {};
  z.x = x;
  z.y = y;
  return z;
};
var z0 = PointA(1, 2);

I've seen several examples illustrating how v8 uses hidden classes to attain fast property lookup, but all these examples employ the standard technique of object construction, namely:

var PointB = function(x, y) {
  this.x = x;
  this.y = y;
};
var z0 = new PointB(1, 2);

If I use a nontraditional factory like PointA am I thwarting the hidden class optimizers of v8?

Answer 1

All JavaScript objects in V8 are using hidden classes machinery, so it does not matter whether you create an object via a constructor or from a literal --- both will have hidden classes.

What matters here however is how hidden classes are connected to each other. Hidden classes are forming transition trees with a root attached to the constructor. If you are creating objects from a literal it means your transition tree is essentially growing from the Object constructor. This can confuse V8 sometimes because transition trees that are logically disconnected seem to be connected to V8 at their root. Take a look at this jsPerf microbenchmark. I create two objects that are seemingly disconnected:

function X() {
  var self = {};
  self.x = 0.1;
  return self;
}

function Y() {
  var self = {};
  self.x = true;
  return self;
}

var y = new Y, x = new X;

Yet internally V8 can't select the most efficient representation for the property x on objects made by X because it also sees objects made by Y as part of the same hidden class transition tree. If you make an object through a constructor that starts a new transition tree and a result there is no type polution.